Diffusion of water in a phase-separated poly(L-lactide)-b-poly(ethylene glycol) (PLLA PEG) copolymer with both blocks crystallizable was investigated using time-resolved ATR-FTIR with deuteratecl water and 2D correlation analysis. Water molecules of three different states were identified in the copolymer matrix, and their formation sequence was determined. Perturbation of water on the copolymer structure was revealed. While the amorphous PLLA chains were not affected, upon water penetration, conformation distribution of crystalline PLLA chains was altered, which can be reversed upon removal of the water, and the PEG crystallinity in the copolymer was gradually destroyed and the process was not reversible. For the double amorphous PLLA PEG film, water first associated with the more hydrophilic PEG chains and then interacted with the PLLA segments. For the double crystalline film, however, crystalline PLLA segments were first perturbed by the water before the crystalline PEG was affected. Finally, the water diffusion coefficient for the PLLA PEG copolymer was measured and compared with that of the PLLA and PEG homopolymers.