Relative densities and relative massic heat capacities have been measured for aqueous solutions of Y(ClO4)(3), Yb(ClO4)(3), Dy(ClO4),, and Sm(ClO4)(3) at T = (288.15, 298.15, 313.15, and 328.15) K and p = 0.1 MPa. These measurements were made in the concentration range 0.01624 less than or equal to m/(mol . kg(-1)) less than or equal to 0.41822 using a Sodev 02D Vibrating Tube Densimeter and a Picker Microflow Calorimeter, respectively. To counter the potential effects of hydrolysis, aqueous solutions of the investigated salts were acidified with perchloric acid. After correcting for the presence of the acid, the measured properties were used to calculate apparent molar volumes and apparent molar heat capacities for solutions of the perchlorate salts in water. The calculated apparent molar properties were modeled at each investigated temperature using Pitzer ion interaction equations to produce estimates of apparent molar volumes and heat capacities at infinite dilution. In addition, the temperature dependences of the apparent molar properties of each salt were found to be well modeled by temperature dependent Pitzer ion interaction equations. The Helgeson, Kirham, and Flowers equations of state were incorporated into the Pitzer ion interaction equations in order to model the infinite dilution properties of the salts. Single ion volumes and heat capacities of the investigated trivalent metal cations have been calculated and compared to previously reported values. (C) 2003 Elsevier Science Ltd. All rights reserved.