A thermodynamic study of monolayer mixing behavior was performed using binary lipid mixtures of DPPC+DOPG (7:1 mol:mol) as well as a mixture containing the phosphocholine acyl chain perdeuterated analog of DPPC, DPPC-d62 (i.e. 7:1 mol:mol DPPC-d62+DOPG). An analysis was performed on these isotherms that calculated the Helmholtz excess free energy of mixing to determine potential thermodynamic differences in the fully protiated DPPC:DOPG monolayer versus the identical monolayer substituted with acyl chain perdeuterated DPPC (i.e. DPPC-d62). The main conclusions of this study are that: (1) the temperature-dependent thermodynamic phase behavior of DPPC-d62 differs substantially from that of DPPC, and these differences appear magnified when these molecules are studied as monomolecular films at the A/W interface as opposed to bulk phase systems; and (2) the calculated excess free energy of mixing values, ΔF(xs)(A), for the deuterium-containing 7:1 DPPC-d62:DOPG binary monolayer film show a type of near-ideal mixing behavior above 20°C that is consistent with the 'squeezing-out' of the DOPG component in the binary mixture containing the DPPC-d62 component. This type of behavior does not occur in the graph of the excess free energy of mixing for the fully protiated 7:1 DPPC:DOPG binary monolayer film. The use of acyl chain perdeuterated DPPC (i.e. DPPC-d62) in binary monomolecular mixtures with other phospholipids produces a real and measurable difference on the thermodynamic properties of the monolayer when compared to the case of the fully protiated monomolecular film. In particular, the presence of DPPC-d62 in a 7:1 mol:mol DPPC-d62:DOPG binary monomolecular film may over-state the fluid nature of the monolayer at any given temperature, thereby leading to an over-estimate of the amount of material potentially 'squeezed out' of the monomolecular film. These results have implications for a commonly used IR spectroscopic method that relies on the incorporation of deuterium-labeled phospholipids into monolayers for vibrational spectroscopic analysis of individual components in a multicomponent monomolecular film. Copyright (C) 1998 Elsevier Science Ireland Ltd.