Ca2+ complexation by both sulfate and selenate ligands was studied by CE. The species were observed to give a unique retention peak as a result of a fast equilibrium between the free ions and the complexes. The change in the corresponding retention time was interpreted with respect to the equilibrium constant of the complexation reaction. The results confirmed the formation of CaSO4(aq) and CaSeO4(aq) under our experimental conditions. The formation data were derived from the series of measurements carried out at about 15, 25, 35, 45 and 55 degrees C in 0.1 mol/L NaNO3 ionic strength solutions, and in 0.5 and 1.0 mol/L NaNO3 ionic strength solutions at 25 degrees C. Using a constant enthalpy of reaction enabled to fit all the experimental data in a 0.1 mol/L medium, leading to the thermodynamic parameters: Delta(r)G(0.1M)(25 degrees C) = -(7.59+/-0.23) kJ/mol, Delta H-r(0.1M) = 5.57+/-0.80 kJ/mol, and Delta S-r(0.1M)(25 degrees C)=44.0+/-3.0 J mol(-1)K(-1) for CaSO4(aq) and Delta(r)G(0.1M)(25 degrees C) = -(6.66+/-0.23) kJ/mol, Delta H-r(0.1M) = 6.45+/-0.73 kJ/mol, and Delta S-r(0.1M)(25 degrees C) = 44.0+/-3.0 J mol(-1) K-1 for CaSeO4(aq). Both formation reactions were found to be endothermic and entropy driven. CaSO4(aq) appears to be more stable than CaSeO4(aq) by 0.93 kJ/mol under these experimental conditions, which correlates with the difference of acidity of the anions as expected for interactions between hard acids and hard bases according to the hard and soft acids and bases theory. The effect of the ionic medium on the formation constants was successfully treated using the Specific ion Interaction Theory, leading to significantly different binary coefficients epsilon(Na+,SO42-) (0.15+/-0.06) mol/kg(-1) and epsilon(Na+,SeO42-) = -(0.26+/-0.10)mol/kg(-1).