Densities of La(CF3SO3)(3)(aq) and Gd(CF3SO3)(3)(aq) have been measured at 278 K, 298 K, and 318 K at pressures of 0.1 MPa, 7.0 MPa, and 30.0 MPa using vibrating-tube densimeters. Apparent molar volumes obtained from the density data have been fitted to the Fitter equation. The standard partial molar volumes V-phi(infinity) and the ion-interaction parameters beta((0)V) and beta((1)V) have been determined through least-squares fitting to the experimental results. The standard partial molar volume of the trifluoromethanesulfonate anion V-phi(infinity)(CF3SO3-,aq) was calculated at 298 K and 0.1 MPa from the partial molar volumes of the gadolinium and lanthanum salts and the available values for the standard partial molar volumes of the cations. Excellent agreement is obtained for V-phi(infinity)(CF3SO3-,aq) calculated from the data for the two salts.