Concentrated dispersions (35% v, v) of ceramic grade kaolin dispersed with sodium hexametaphosphate (NaHMP) were prepared at three different experimental conditions: in distilled water at 30 and 60 degrees C, and in a 5 * 10(-3) M Ca(NO3)(2)(.)4H(2)O solution, at 30 degrees C. The dispersions were kept at constant temperatures for different ageing times (2, 4, 6, 18, 24, 48 h). After each ageing period they were submitted to rheological measurements and centrifuged to measure the concentration of HMP- ion in the supernatant solution by ICP. Measurements of the electrophoretic mobility in dispersions containing Ca(NO3)(2)(.)4H(2)O and KCl as background electrolyte were also performed both in the presence and in absence of NaHMP. The results allow a better understanding of the roles played by calcium ions and ageing of kaolin dispersions in the presence of NaHMP. They also shed light on the deflocculating mechanism of NaHMP. Correlations between theological parameters and chemical properties of the dispersions could be established.