Some factors that influence the acid gelation of milk during heat treatment were investigated. The effect of varying the formation of soluble complexes by fixing the casein micelles and adjusting the pH before heat treatment was studied by rheology and size exclusion chromatography. The behavior of aggregation and particle size of the soluble complexes, isolated from the milk and concentrated, were studied by dynamic light scattering (DLS), at different pH and temperatures over time. The isolated soluble complexes were observed by cold-field-emission scanning electron microscopy. The results showed a positive correlation between the amount of soluble complexes present in the serum phase of the milk and final G' of acid gels, with a higher G' when more soluble complexes were formed. These particles appear to start aggregating at pH 5.2 as shown by DLS. Their aggregation behavior seems to be very unstable and appears to be time and temperature dependent.