Pentacosadiynoic acid (PDA) monolayers on aqueous subphase containing Ca2+ and on water subphase (Langmuir (L) film) were irradiated by UV light, and the reaction of the L films was monitored by surface area change under constant-pressure condition and by surface pressurechange under constant-area condition. The highest rate of reaction was obtained at a surface area of ca. 28 Á2/molecule, indicating that the PDA molecules have the most suitable molecular arrangement in themonolayer. The change of the optical absorption spectra of the L films also supports this finding. It was also found that the arrangement of the molecules in the Langmuir-Blodgett (LB) film is related to that of the monolayer from which the LB film was prepared.