Osmotic coefficients of aqueous solutions of tris(ethylenediamine)cobalt(III) oxalate have been obtained from 0.000 to 0.010 mol.kg-1 by the freezing-point method. Apparent molar volumes have been determined by density measurements of aqueous solutions of tris(ethylenediamine)cobalt(III) perchlorate at 288 and 278 K in the range 0.000-0.060 mol.kg-1. Partial molar volumes at infinite dilution for the complex cation at 288, 278, and 273 K are estimated.