Mixed monolayers of an acylated derivative of hepatitis A synthetic peptide VP3(110-121) with neutral, cationic or anionic lipids were spread at the air/water interface. Deviations from ideality as well as thermodynamic values were calculated at different surface pressures using the free-excess energy, the interaction parameter and the enthalpy. The miscibility at the collapse point was also checked. Maximum deviations from ideality were found for mixtures containing the anionic lipid phosphatidylglycerol (PG), and it seems that the monolayer composition is not stable through compression, as the peptide is ejected from the film. Films containing neutral [dipalmitoylphosphatidylcholine (DPPC)] or cationic [stearylamine (SA)] lipids showed more regular behaviour. As the peptide has a net negative charge it is probable that electrostatic interactions are in part responsible of the good miscibility of palmitoyl VP3(110-121) with SA. In order to prepare liposomes containing palmitoyl VP3(110-121), lipids such as SA or DPPC/SA will be a more suitable choice than anionic lipids such as PG.