Relative densities have been measured for acidified aqueous solutions of ytterbium perchlorate {Yb(ClO4)(3)} at approximately T = (348.15, 373.15, 398.15, and 423.15) K and p = (10.0, 20.0, and 30.0) MPa over the concentration range 0.01624 less than or equal to m(2)/(mol (.) kg(-1)) less than or equal to 0.2531 using an optically coupled vibrating tube densimeter (OCVTD). Experimental apparent molar volumes have been calculated from the density measurements, and apparent molar volumes for the aqueous perchlorate salt have been calculated using Young's rule. The application of Young's rule requires apparent molar volumes for aqueous perchloric acid (HClO4) solutions over extended temperature and pressure ranges. These values were calculated from densities for aqueous HClO4 solutions that were measured using the OCVTD at the same temperatures and pressures as those used to investigate the density surface of the acidified aqueous Yb(ClO4)(3) solutions. The temperature, pressure, and composition surfaces of the apparent molar volumes for Yb(ClO4)(3)(aq) and HClO4(aq) have been modelled using Pitzer ion-interaction equations. Apparent molar volumes at infinite dilution obtained from these models have been compared to those which can be calculated using the semi-empirical Helgeson, Kirkham, and Flowers equations of state. Values for the apparent molar volume at infinite dilution of the ytterbium trivalent cation have also been calculated using simple additivity principles. (C) 2004 Elsevier Ltd. All rights reserved.