In this paper, we compared the interfacial behaviors of spread films of poly(ethylene glycol) (PEG2000), poly(D,L-lactide) (PLA50), and a mixture of PEG2000 and PLA50, to understand the properties of a PEG2000-PLA50 diblock copolymer at the air/water interface. This was achieved (i) by analyzing the surface pressure-surface area curves obtained on a Langmuir trough and (ii) by modeling the dilatational properties of the films according to a modified Maxwell model. The properties of the films composed of a mixture of PEG2000 and PLA50 showed the influence of PEG2000 before the nucleation transition of PLA50. Then, increasing the lateral compression of the mixed monolayer led to the expulsion of PEG2000 segments with presumably their irreversible desorption into the bulk. In the case of the copolymer, the covalently bound PEG2000 segments were segregated at low surface coverage forming a stable film with the PEG corona oriented toward the water phase. This situation was conserved until high surface coverage (10-15 mN/m). Afterward, compression led to the penetration of PEG2000 into the tridimensional layer oriented toward the air/phase. These results allow better understanding and modulation of the composition and the hydrophilic character of interfaces formed during emulsion processes and thus improvement of the control of the surface properties of drug delivery systems.