Abstract The monolayer properties of mixtures of poly(octadecyl methacrylate) and poly(butyl methacrylate) have been compared with those of their corresponding copolymers of octadecyl methacrylate and butyl methacrylate, using the measurements of their surface pressure-area isotherms at the air/water interface at 20 °C. The results showed that the isotherms of the mixed monolayers had three break points, and the surface pressures of the lower two break points agreed with those of poly(butyl methacrylate). This suggests that at lower surface pressures, the mixtures of the two homopolymers are compatible in a bidimensional state when each phase contains a small amount of the other homopolymer, and otherwise these are incompatible, while at higher surface pressures, up to 70 segment mol% of poly(octadecyl methacrylate) they are incompatible and above 80%, they are compatible. The copolymers showed characteristic isotherms different from those of the mixtures. The properties of the copolymers in the bidimensional state are affected by variations in the vitrified, liquid, or crystalline states with an increase in the mean lengths of the side chains in the three-dimensional state.