Heat-induced aggregation and gelation of aqueous solutions of whey protein isolate (WPI) in the presence of sodium caseinate (SC) and CaCl2 was studied at pH 6.6. The effect of adding SC (0-100 g/L) on the structure of the aggregates and the gels was investigated by light scattering and confocal laser scanning microscopy at different CaCl2 concentration ([CaCl2] = 0-30 mM). The gelation process was studied by oscillatory shear rheology. At the whey protein concentrations studied here (34 and 60 g/L), no gels were formed in the absence of CaCl2 and SC. However, WPI solutions gelled above a critical CaCl2 concentration that increased with increasing SC concentration. In the absence of CaCl2, WPI gels were formed only above a critical SC concentration. The critical SC concentration needed to induce WPI gelation decreased weakly when CaCl2 was added. In an intermediate range of CaCl2 concentrations, gels were formed both at low and high SC concentrations, but not at intermediate SC concentrations. Finally, at high CaCl2 concentrations gels were formed at all SC concentrations. The gelation rate and the gel structure of the gels formed at low and high casein concentrations were very different. The effect of SC on the thermal gelation of WPI was interpreted by competition for Ca2+, a chaperon effect, and microphase separation.