Static and dynamic light scattering studies of sodium caseinate solution in a Pipes-KCl buffer at pH 6.75 and a constant ionic strength in the dilute concentration regime are presented. In the absence of calcium ions, a bimodal distribution with comparable scattering intensity contributions has been consistently observed by using the CONTIN method to analyze the dynamic light scattering data. However, based on the weight fraction of submicelles and that of the aggregates, the casein solution remains overwhelmingly in the submicellar form with sizes in the 20-nm range. The calcium-induced aggregation of these casein submicelles appears to be a time-dependent process. After reaching equilibrium, two aggregate fractions of different average sizes (about 90 and 350 nm) have been detected in the casein solution. Casein submicelles are generally stable at high temperatures. But, a prolonged ultrahigh-temperature treatment (10 min at 140 degrees C) can fully transform casein submicelles into very large aggregates. Calcium-induced aggregates exhibit reduced thermal stability. Modification of casein with iodoacetic acid to block the SH groups gives the protein a stronger resistance to calcium-induced aggregation, but significantly reduces its stability at high temperatures when the calcium ions are present.