The reconstituted dialkyl viologen monolayer at the air/water interface was studied as a model system of the biomembrane for the study of lateral diffusion of the electroactive viologen moiety because lateral diffusion of biomembrane molecules has key functions for the life phenomena. The shortest alkyl chain of 1,1'-didodecyl-4,4'-bipyridium dibromide (C12VC12) could not form the stable monolayer in the subphase of NaCl and NaBr aqueous solutions and also does not show any distinct phase transitions because it is partially soluble in the aqueous phase. For 1,1'-ditetradecyl-4,4'-bipyridium dibromide (C14VC14) and 1,1'-dihexadecyl-4,4'-bipyridium dibromide(C16VC16) molecules, the stable monolayer was obtained at the air/water interface, and this was identified with pressure-area isotherms and directly observed at Brewster angle microscopy (BAM) images. The changing orientation of alkyl chain of the viologenes was detected by surface potential measurements with moving barrier. Finally, the lateral diffusion of the viologen moiety of dialkyl viologenes at the air/ water interface was successfully determined with electrochemical techniques using a microband electrode. The linear decrease of the diffusion constant of viologen molecules with decreasing mean molecular area and increasing alkyl chain length of viologenes at the air/water interface is explained by the decreasing free volume of the moving headgroup. This is well explained by the modified Cohen-Turnbull free volume model.