To better evaluate the role of sialic acid residues in transferrin saturation, two different forms of human serum transferrin, namely the apo and the corresponding apo desialylated forms, were saturated with Fe3+ and the heats of saturation measured at 298.15 K. In both cases an exothermic reaction was observed, the value being larger for the apo form. The presence of sialic acid is believed to facilitate the saturation process in human serum transferrin.