An investigation has been made of the interaction between chlorpromazine (CPZ) and monolayers of 1,2-dipalmitoyl-sn-3-glycerophosphatidylcholine ( DPPC) and 1,2-dipalmitoyl-sn-3-glycero[phospho-rac-(1-glycerol)] ( DPPG), both at the air/water interface and in transferred Langmuir-Blodgett films. The Gibbs free energy, Delta G, and the compressibility modulus (C-S(-1)), obtained from the surface pressure isotherms, indicated changes in the in-plane interactions of CPZ/DPPG mixed monolayers, with positive values of Delta G. The arrangement of CPZ in the zwitterionic DPPC monolayers causes a weaker interaction in CPZ/ DPPC mixed monolayers, with the Delta G fluctuating around zero. IR measurements in transferred monolayers showed that CPZ did not affect the conformational order of the acyl chains, its effects being limited to the bands corresponding to the headgroups. Furthermore, since no shift was observed for the acyl chain bands, the phase transition induced by CPZ is not a liquid expanded (LE) to liquid condensed (LC) transition, as the latter is associated with chain ordering. Taken together, the IR and compressibility results demonstrate that the effect from CPZ cannot be correlated with temperature changes in the subphase for pure monolayers, in contrast to models proposed by other authors.