The influence of sodium caseinate (SC) on heat induced gelation of whey protein isolate (WPI) was studied in pure water and in 0.1 M NaCl at pH 5.8, 6.0, 6.3 and 6.6. The critical gel concentration (Cg) was determined for pure protein solutions and for mixtures containing 40, 60 or 80% SC. Analysis of the aggregates that were formed by the heated mixtures at C < Cg showed that SC did not aggregate, but did influence the aggregation process of WPI. When the fraction of SC in the mixture was less than about 50% it inhibited WPI aggregation, but when it was higher it led to formation of larger aggregates and a reduction of the WPI concentration needed to form a gel. Oscillatory shear measurements were done to study the gelation process and the gel microstructure was investigated with confocal laser scanning microscopy.