Whey protein isolate was modified by proteolysis using a broad-spectrum protease in combination with heat treatment of the hydrolysate. Half of the beta-lactoglobulin content was hydrolyzed when the degree of hydrolysis reached 5.1%. alpha-Lactalbumin and BSA were not attacked by the enzyme. Heating of the hydrolysate resulted in the formation of small and large aggregates, the proportion of which depended on the degree of hydrolysis and the pH during heating. The decrease in total sulfhydryl groups, as the degree of hydrolysis and heating pH increased, was associated with the formation of disulfide bonds. Whey protein hydrolysis at a degree of hydrolysis of 1.7%, followed by heating at pH 8.0, resulted in the highest amount of accessible sulfhydryl groups, reflecting the unfolded structure of the aggregates. Hydrolysate solubility at neutral pH averaged 98% when pH during heating was 4.0 or 8.0. Heating of whey protein at pH 6.0 resulted in a much lower solubility, which was attributed to the high proportion of large aggregates. Solubility of the hydrolysate at pH 4.5 was higher when pH during heating was adjusted to 4.0. Solubility of the hydrolysate heated at pH 6.0 and 8.0 improved with degree of hydrolysis > 1.7%. Results are discussed in relation to interactions of proteins and peptides during heat processing.