Symmetrical or unsymmetrical dyes of the general formula <FORM:0646123/IV (c)/1> wherein m and n are 1 or 2, Y and Z are the non-metallic atoms to complete carbocyclic or heterocyclic rings, which may have fused-on rings attached thereto, and X is the non-metallic atoms to complete a carbocyclic or heterocyclic ring with any 2 carbon atoms of the polymethine chain, wherein X, Y and Z may form identical or different rings, wherein the remaining H-atoms of the polymethine chain may be replaced by alkyl, aryl, aralkyl, acyl, or carboxyethyl groups, e.g. acetyl, p-iodophenyl, or 2-pyridyl groups, and wherein the hydrogen of the enol group may be replaced by metals (e.g. Na, K, Ca, or Ag), triethylammonium or acetyl, are prepared by the condensation in molecular proportion of carbocyclic or heterocyclic 5- or 6-membered compounds containing a reactive methylene group next to a CO group with compounds of the general formula <FORM:0646123/IV (c)/2> wherein V is O, N-aryl, or (O-alkyl)2 and W is OH, ONa O-alkyl, O-CO-alkyl, O-CO-aryl, halogen, or a substituted amino group or, where Y and Z complete identical rings, by condensing two moles of the first reactant with one mole of a compound of the general formula <FORM:0646123/IV (c)/3> or, where X, T and Z complete identical rings, by condensing three moles of the first reactant with one mole of a compound of the general formula V=CH-W. X may be butylene, o-phenylene, or complete a pyrazole or isoxazole ring. The first reactant may be 1 : 3-indanedione, 3-phenyl-5-isoxazolone, 3-methyl-5-pyrazolone, 1 - phenyl - 3 - methyl - 5 - pyrazolone, 1 : 11 - (diphenylen - 4 : 41) - bis - (3 - methyl-5-pyrazolone), barbituric acid, thiobarbituric acid, rhodanine, phenylrhodanine, pseudothiohydantoin, and derivatives such as sulphonic acids. Compounds of general formula (A) or (B) specified are 4 : 41-methenyl-bis-1-phenyl-3-methyl-5-pyrazolone and its disulphonic acid, bi-indone, methenyl-bis-indanedione, and 1-phenyl - 3 - methyl - 4 - (3 - oxo - 2 - phenyl-hydrindyliene) - pyrazolone - 5. Compounds of general formula (C) specified are hydroxymethylenecyclopentanone, hydroxymethylenecyclohexanone, a 5 - pyrazolone - 4 - aldehyde such as a 1-aryl-3-alkyl-5-pyrazolone-4-aldehyde or compounds wherein the O of the aldehyde group is replaced by =N-aryl or (O-alkyl)2, 2-hydroxythionaphthen-3-aldehyde, hydroxymethylenecamphor, hydroxymethylenedihydroisophorone, 1 : 11-dimethylcyclohexane-3 : 5-dione, ethyl 1 : 3-indanedione-2-carboxylate, benztetronic acid, 1-chlorocyclohexanone-3, 1-benzoyloxy-3-oxo-2-phenylindene, derivatives of homophthaldehyde such as o-formyl-styrylaminosulphonic acid, and the substituted glutaconic dialdehydes obtained by condensing chlorosulphonic acid or 2 : 4-dinitro-chlorobenzene with pyridine derivatives substituted by side-rings, such as cinchomeronic anhydride and cinchomeronimidine. Compounds of the general formula V=CH-W specified are diphenylformamidine, diphenylacetamidine, and ethyl orthoformate. Compounds readily splitting off the compound V=CH-W, e.g. 1-phenyl-2 : 3-dimethyl-5-pyrazolone - 4 - aldehyde, 2 - hydroxynaphthyl-1 - aldehyde, and a - methylindole - b - aldehyde may be used. Indanedione derivatives may be replaced by perinaphthindanedione, 8 - phenyl - perinaphtindanedione - 7 : 9, or derivatives obtained from naphthalic anhydride or nitronaphthalic anhydride and p-nitro-phenyleneacetic acid. Dimalonylperylene may be used as starting material. Dyes may be obtained from the pyrazolone from dehydrothiotoluidine monosulphonic acid and 1-phenyl-3 - methyl - 5 - pyrazolone - 4 - carboxylic acid, and dyes containing three 1 : 11-(di-phenylene-4 : 41) - bis - (3 - methyl - 5 - pyrazolone)-nuclei may be formed. In examples, dyes are obtained (1) by heating a methanol solution of KOH, 1 - phenyl - 3 - methyl - 5 - pyrazolone, and 4 : 41 - methenyl - bis - (1 - phenyl - 3-methyl - 5 - pyrazolone), the purple-red enol dye obtained being convertible into an orange-red ketone dye by acidification; (2) as (1) using 1 - p - sulphophenyl - 3 - methyl - 5-pyrazolone; (3) by acetylating the enol dye of (1) by heating with acetic anhydride and dry sodium acetate; (4) by heating a methanol solution of 1 - phenyl - 3 - methyl - 5 - pyrazolone, 4 : 41 - methenyl - bis - 1 - p - sulphophenyl - 3 - methyl - 5 - pyrazolone, and aqueous ammonia; (5) as (4) using NaOH in place of aqueous ammonia to give a pentamethine dye; (6) by heating a methanol solution of the ammonium salt of 1-p-sulphophenyl - 3 - methyl - 5 - pyrazolone and sodium hydroxymethylenecyclohexanone; (7) as (6) using hydroxymethylenecamphor; (8) by heating a methanol solution of 1-phenyl-3-methyl-5-pyrazolone, 1 : 1 - dimethylcyclohexane-3 : 5-dione, and triethylamine; (9) by heating together 1 - phenyl - 3 - methyl - 5 - pyrazolone and sodium 1 : 3 - indanedione - 2 - carboxylate; (10) by reacting together in warm water the sodium salts of 1 : 3-indanedione-2-carboxylic acid and 1-p-sulphophenyl-3-methyl-5-pyrazolone, and heating with a further quantity of the latter salt; (11) by heating an ethanol solution of diphenylformamidine, 1-phenyl - 3 - methyl - 5 - pyrazolone, and KOH (a mixture of ether and alcohol as solvent gives the intermediate product 4 : 41-methenyl-bis-(1 - phenyl - 3 - methyl - 5 - pyrazolone); and (12) by heating a methanol solution of phenylisoxazole, diphenylformamidine, and KOH. Specifications 506,385, [Group XX], and 515,998, [Group IV], are referred to. The Specification as open to inspection under Sect. 91 comprises also the subject-matter of Specification 646,125, [Group XX]. This subject-matter does not appear in the Specification as accepted.