The self-diffusion of water and a small and a large poly(ethylene glycol) (PEG) were monitored during the sol-gel transition of a casein system induced by chymosin action using pulsed field gradient (PFG) NMR techniques throughout the coagulation process. Rheological measurements revealed that the diffusion rate of the molecules studied was unaffected by the establishment of a network. However, different evolutions occurred during the gel aging phase, depending on the size of the diffusing molecule. The diffusion coefficient of water remained constant whereas the diffusivity of the small PEG decreased and that of the large PEG increased. Scanning electron microscopy images showed that all these findings could be explained by the progressive compaction of the casein network caused by the occurrence of structural rearrangements. This study demonstrates the sensitivity of probe diffusion to structural changes in casein gels and also the potential of PFG-NMR techniques to reveal dynamic information on evolving systems at different length scales.