Vibrating tube densimetry was used to investigate volumetric properties of MgCl2(aq) at target molalities between 0.005 mol.kg(-1) and 3 mol.kg(-1), at temperatures from 298 K to 623 K and pressures up to 30 MPa. The new data are correlated at constant T and p as a function of molality by the Fitter equation in order to obtain partial molar volumes at infinite dilution. Volumes for MgCl2(aq) as a function of temperature and pressure are compared with those for other basic types of strong electrolytes. Data on association constants, volumes of ion-pairing, and activity coefficients, derived from information available in the literature, were used to calculate the degree of association in MgCl2(aq) as a function of molality and temperature. A modified form of the Fitter equation was used to obtain a semiquantitative estimate of the partial molar volumes at infinite dilution for the ionized salt.