Caseins are milk's major proteins. They are present in milk as a suspension of particles called casein micelles. In the dairy industry, two classical processes which are acid and rennet coagulation or a combination of both are used to convert milk into gels, such as yoghurt or cheese curd. Acids gels are obtained through the acidification of the medium whereas rennet gels are formed after enzymatic hydrolysis. It is widely accepted that casein gels are so-called particle gels (Lucey J.A. J. Dairy Sci., 85, 2002) that is that they are formed by aggregation of casein micelles. Nevertheless, it is well known that gels made by rennet or/and acid action do not have the same rheological and structural properties. For example, rennet gels are more viscous than their acid counterparts and would be constituted of larger aggregates and pores in between (Aichinger P.A. et al., Colloid Surf. B-Biointerfaces, 31, 2003). As a consequence, functional properties such as water holding capacity or permeability are different within rennet and/or acid gels (Lucey J.A. et al., Int. Dairy J., 11, 2001; Mellema M. et al., Langmuir, 16, 2000), but at the same time, still very little is known about the impact of the coagulation type on diffusion. Previous works have shown that diffusion is slower in liquid milk than in rennet gels and that diffusion rate is strongly dependent on the molecular size of the diffusing probe (Colsenet R. et al., Macromolecules, 38, 2005). The purpose of this work was to study the diffusion of solutes in acid casein gels in order to achieve a better understanding of the microstructure effect. Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) was used to investigate the self-diffusion behavior of polyethyleneglycols (PEG) in caseins suspensions and acids gels at different concentrations. Four PEGs with different molecular mass (from 1010 to 520500 g/mol) at a concentration of 0,1 % (w/w) were used. The polydispersity of the polymers were taken into account using the method of cumulants. Glucono-delta-lactone (GDL), an acidifying agent was used to achieve the acidification and acid coagulation. The dry matter effect of the GDL addition was taken into account to calculate the reduced coefficient of diffusion D/D0. The self-diffusion proved to be independent of diffusion distance which means that no restricted diffusion occurred. Parts of the results are presented in the figure above. The reduced self-diffusion coefficients D/D0 of the different PEGs, in suspensions and in gels, are reported as a function of the casein concentration. As illustrated in the figure, the mobility of polymers is reduced, in both solution and gel, with increasing polymer molecular weights and protein concentration. It also appears that the diffusion of the two biggest PEGs is faster in acids gels as compared to that of suspensions. All these results are similar to those obtained for rennet gels. Thus, it appears that the diffusion coefficient is not or slightly sensitive to the type of coagulation and therefore to the structure of casein aggregates. The obstruction model used to describe the diffusion of PEGs in rennet gels will be confronted to the data obtained with acids gels.