Very little is known about the behaviour of soy proteins when used as ingredients in foods despite of the increase in the consumer demand of products containing soy proteins. This research focused on the effect of the addition of soy proteins to milk proteins during heating and acidification. Mixed systems containing soy proteins and milk proteins were studied under different processing conditions and at different soy to milk protein ratios. The systems were characterized using dynamic light scattering, confocal microscopy, rheology, size exclusion chromatography, electrophoresis and multi angle light scattering detection. The addition of soy proteins had an effect on the aggregation behavior and gelation of milk proteins. During acidification with glucono-[delta]-lactone, soy protein affects the pH onset of aggregation, network structure and the elastic modulus of the casein network. When whey proteins were replaced by soy proteins in a milk system, soy proteins affected the aggregation and gelation behavior of the casein micelles, but to a less extent than the heated whey proteins. The mechanism of aggregation of casein micelles containing soy proteins, however, showed different properties from the acid casein gel containing whey proteins. It was shown that mixed systems containing soy proteins and whey proteins form aggregates during heat treatment and their aggregation behavior is different from that of the proteins heated in isolation. The type of aggregates formed in these soy/whey mixes was influenced by the soy to whey protein ratio, temperature and heating time. At higher soy to whey protein ratios soluble aggregates were formed. On the other hand, in the systems containing soy and sodium caseinate treated at different temperatures or after homogenization, we were not able to identify process-induced protein-protein interactions. When soy protein were incorporated to the casein micelle-soy mixture, the mixtures showed similar characteristics to the controls suggesting to it is possible to incorporate soy proteins until a critical level without modifying the properties of the acid gels. In skim milk/soy protein systems soy proteins may interact with whey proteins and to a lesser extent with casein micelles and the interactions are influenced by amount, processing history of the soy proteins and other components present in soy proteins.