In continuation of the previous work on analgesics, 10 kinds of 5-substituted 1-(2-diethylaminoethyl)-2-(p-ethoxyphenylthio) benzimidazole were synthesized and tested but none of them showed any marked effect. Some of the previously prepared 5-unsubstituted 1-(2-tert-aminoethyl)-2-(P-substituted phenylthio) benzimidazoles showed a powerful analgesic effect. Comparison of infrared and nuclear magnetic resonance spectra of these two series of compounds showed a distinct difference between them in the absorption considered to be due to CH₂ neighboring the nitrogen in diethylaminoethyl group in 1-position, the former showing absorption at around 2800 cm^-1 which is absent in the latter. The nuclear magnetic resonance signal of the latter is shifted to a lower magnetic field than that in the former. These evidences suggest a difference in the steric structure of the diethylaminoethyl group in 1-position of the two series of compounds and this difference may be related to their pharmacological effect.