PEG self-diffusion coefficients of poly(ethylene glycol)s (PEGs) (1080, 8500, and 82 250 g/mol) were measured by PFG-NMR spectroscopy in whey protein solutions and gels in relation to whey protein concentration effects (from 6.49 to 40.45 g/100 g) and whey protein heat denaturation effects (30 min at 70 degrees C). A strong dependency of diffusion on probe size was observed in both whey protein solutions and gels: as PEG size increased, diffusion was reduced. This effect was more pronounced for higher protein concentrations. Changes in whey structure after thermal aggregation increased the diffusion coefficient for all PEGs, particularly for the 8500 g/mol PEG. The PEG self-diffusion coefficients in whey protein gels were compared to the gel structures characterized by scanning electron microscopy. The results are discussed in relation to a reptation model and compared to PEG diffusion in casein micelle suspensions and gels.