Mixed monolayers of CyA and poly(DL-lactic acid-co-glycolic acid) spread at the air/water interface were studied by measuring surface pressure (pi) as a function of the area per residue (A). The pi-A curves suggest that the components are miscible below surface pressures of 10 mN/m. Also, the diagrams obtained by plotting the mean area per residue as a function of molar fraction show positive deviations from ideal, which indicate the existence of interactions between both components in the mixed films. The nature of these interactions was influenced by the pH and temperature of the subphase; however, it was not dependent on the molecular size of the polymer. Above pi = 10 mN/m mixed monolayers showed immiscible behavior with no interactions between their components. This was manifested by the obedience to the mean area additivity rule and the same value of collapse pressure present for all mixed monolayers studied.