Lateral diffusion of alkylferrocenecarboxamide(FcCONHC(n))/1,2-dilauroyl-sn-glycero-3-phosphatidylcholine lipid monolayer at the air/water interface was determined with electrochemical techniques using a fabricated microline electrode. The prepared homogeneous Langmuir monolayer of electroactive FcCONHC(n)/ phospholipid was identified with pressure-area isotherms in the subphase of 50 mM HClO4. The diffusion constant of electroactive FcCONHC(n) in the lipid monolayer was approximately between 0.1 x 10(-6) and 2.6 x 10(-6) cm(2)/s at room temperature. The diffusion constant of ferrocenes in a phospholipid monolayer at the air/water interface was greater for the ferrocene with longer alkyl chains. This is interpreted as the extent of hydrodynamic coupling of the hydrophilic ferrocene moiety of FcCONHC(n) to the subphase, which was less for ferrocenes with longer alkyl chains. This lets the position of the hydrophilic headgroup of FcCONHC(n) with respect to the air/water interface be shifted upward for the longer alkyl chain because of greater hydrophobic entanglement. The electrochemistry with the microline electrode was possible in the mean molecular area range 60-100 Angstrom(2)/mol because of monolayer adsorption in a mean molecular area that is less than 60 Angstrom(2)/mol. The linear decrease of the diffusion constant of ferrocenes with decreasing mean molecular area is explained by the decreasing free volume of the moving headgroup. This is well explained by the modified Cohen-Turnbull free volume model.