The self-diffusion coefficients of 620 and 96 750 g/mol poly(ethylene glycol) (PEG) were measured by pulsed field gradient (PFG)-NMR throughout the coagulation processes of a concentrated casein suspension induced by acidification alone and with the concomitant action of chymosin. The diffusion of water molecules was also investigated during the acidification process. In all experiments, the diffusion coefficient of the molecule studied was modified nonlinearly, and the precise moments at which its evolution changed corresponded to key stages of both coagulation processes. These variations were not directly related to sol-gel transitions as revealed by theological measurements but were caused by other types of structural changes in the sample whatever its viscoelastic state (solution or gel). The diffusion of the large PEG was very sensitive to variations in the size of the casein particles and the casein aggregates constituting the network before and after gelation, respectively. In contrast, the diffusion of the small PEG and of water was sensitive to changes in the internal structure of the colloidal matter. The evolution of the sample microstructure could thus be monitored by diffusion measurements throughout the coagulation processes. In comparison, rheometry, stiffness measurements, and scanning electron microscopy revealed these modifications in the gel state only.