The mixed monolayers of dipalmitoyl phosphatidylcholine (DPPC) with 3-monopalmitoyl glycerol (PG) and palmitic acid 4-methylumbelliferyl ester (4-MU) were compared. Relevant thermodynamic parameters such as excess area (Delta A(E)) and excess free energy of mixing Delta G(mix)(E) were derived from the surface pressure data obtained from compression measurements performed in a Langmuir trough. Generally, DPPC formed more condensed monolayers with PG and greater negative values of Delta A(E) and Delta G(mix)(E) were observed for DPPC/PG system than for DPPC/4-MU one. The positive values of the excess free entropy of mixing (Delta S-mix(E),) were calculated for DPPC/4-MU system at lower temperatures and for DPPC/PG system at higher temperatures. (c) 2007 Elsevier B.V. All rights reserved.