Interfacial properties of spread monolayers of poly(lactic acid), PLA and poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) block copolymer (Pluronic6400) have been studied at the air–water interface by measuring surface pressure–area isotherms. Film balance experiments were also performed on the mixed layers of PLA and Pluronic6400 over a wide concentration range. Transitions observed of the isotherms compressing the pure Pluronic6400 monolayer could be correlated to conformational changes of other PEO-containing type copolymers interpreted by the scaling theory. In the mixed systems, the Pluronic6400 additive exhibited a concentration-dependent influence on the behaviour and structural changes of the PLA film. The maximum effect on phase transition of PLA monolayer and compressibility was obtained at 100∶60 PLA/Pluronic6400 weight ratio. Analysis of the additivity of the components revealed that the mixed films are miscible and non-ideal systems with high deviation from ideality. The interaction existing in the PLA–Pluronic6400 monolayer results in contraction in the low surface pressure range where the film is characterized by low compressibility, while in the compressed state dynamic effects might also contribute to the expansion of the mixed films.