By studying the diffusion of probe molecules of various sizes, information can be obtained on the microstructure of a sample at different length scales. The diffusion of poly(ethylene glycol)s (PEGs) measured by pulsed field gradient (PFG)-Nuclear Magnetic Resonance is probably the most widely used method to perform these investigations. PFG-NMR is a very powerful and nondestructive technique to determine self-diffusion coefficients, and PEG molecules selected as probes offer several advantages. They are water-soluble and available in a wide range of molecular weights with low polydispersity indices, and their NMR signal is a sharp band. Moreover, PEGs present very weak interactions with proteins. PEG diffusion determined by PFG-NMR techniques makes possible the observation of obstruction effects in real biological matrices. In the present study, we illustrated the potentiality of the PFG-NMR technique to investigate structural changes in dairy protein gels, and the sensitivity of probe diffusion to reveal dynamic information on evolving systems at different length scales. The NMR experiments were performed on highly concentrated casein system and the three different coagulation processes were studied: a chymosin coagulation, a coagulation induced by acidification alone and with the concomitant action of chymosin. The self diffusion of a small and a large PEG were investigated by PFG-NMR throughout each type of coagulation in order to probe the microstructure at different length scale. Probe diffusion in casein suspensions and gels is greatly dependent on both the volume fraction occupied by casein particles and the probe size. The reduction in diffusion coefficient for a given volume fraction of casein particles is smaller for smaller probes. This phenomenon was explained by assuming a model with two diffusion pathways, one around and one through the casein micelles. According to the two site model, variations in the diffusion rate of a molecule only depend on its ability to diffuse through the casein particles and the volume fraction occupied by them. This model therefore implies that the diffusion of larger molecules is more affected by the presence of casein particles because they can less easily diffuse through them. This means that the through the casein particles diffusion component for the large PEG (96750 g/mol) is much smaller than those for water and the small PEG (620 g/mol). This model could be used to explain the self-diffusion coefficients of the 96750 g/mol and the 620 g/mol PEG measured during the chymosin coagulation process (Figure 1). Two different phases clearly appeared. During the first period (from t = 0 to about 5 h) the diffusion coefficient of the polymer remained stable while the transition from solution to gel occurred between approximately t = 2 and 3h as revealed by rheological measurements (data not shown). The gel time was found to be tgel= 2 h 35 ± 3 min. During the second period the diffusion coefficient of the larger PEG increased regularly with time, while a small decrease was observed for the smaller PEG size. The changes in PEG diffusion rates observed is a consequence of the rearrangement processes that occur during gel ageing, because this large PEG is unable to diffuse through casein particles. The self-diffusion coefficient of this polymer can only increase because the volume fraction that is accessible to this probe increases when the volume occupied by the obstructing elements decreases. The results presented clearly demonstrate that the diffusion rate of the 96750 g/mol PEG was very sensitive to the progressive increase in gel porosity induced by particle fusion and inter-particle rearrangements. On the other hand, the through the casein aggregates diffusion component would be expected to decrease as they become denser, and this lead to a reduction in the diffusion rate as it is observed for le smaller PEG. The evolution of the 620 g/mol PEG self-diffusion coefficient seems therefore to be sensitive to phenomena occurring inside the casein aggregates and thus to their structure. We revealed by an original designed experiment that probe diffusion measurements can give dynamic information regarding the structural modifications that take place during the gel ageing phase of a casein coagulation process. Other PFG-NMR experiments done during the coagulation processes induced by acidification alone and with the concomitant action of chymosin will be presenting to highlight the sensitivity of this approach.