Densities of Gd(CF3SO3)(3)(aq) solutions with molalities between (0.058 and 0.72) mol kg(-1) were measured with a vibrating-tube densimeter at the temperatures (373, 423, and 473) K and pressures (7 and 26) MPa. Apparent molar volumes calculated from measured densities in this work and those at ambient conditions reported previously by us were represented with the Pitzer ion-interaction treatment. The temperature and pressure dependence of the standard partial molar volume and the virial coefficients in the Pitzer equation were expressed by empirical functions in which the compressibility of water was used as an independent variable. The conventional standard partial molar volume V-m(O)(Gd3+, aq) from T = 278 K to T = 473 K was calculated from V-2,m(O){Gd(CF3SO3)(3), aq} and V-m(O)(CF3SO3-, aq).