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Osmotic coefficients have been determined by the isopiestic method for aqueous solutions of Mn(C104)2, Co-(0104)2, Ni(C104)2, and Cu(C104)2 at 25°. Activity coefficients for Co(C104)2 and Ni(C104)2 are calculated.The results are compared with the data for Mg(C104)2 and Zn(C104)2 of Robinson and Stokes. The osmotic coefficients of equally concentrated solutions of Mn(C104)2, Co(C104)2, Ni(C104)2, and Zn(C104)2 are found to be the same, within the experimental error, up to approximately 3.0 m, while they are slightly lower for Mg(C104)2 and Cu(C104)2 at concentrations exceeding 1.0 m. Absorption spectra of Co(C104)2 and Ni(C104)2 solutions are found to be independent of concentration, while the spectrum of Cu(C104)2 shows small changes which may be ascribed to the formation of an outer-sphere complex. Taking into account both osmotic and absorption spectra results, the conclusion is drawn that octahedral hexaaquo complexes, probably with second layers of hydrogen-bonded water molecules, are maintained unchanged in the whole concentration range investigated in solutions of Mn(C104)2, Co(C104)2, Ni(C104)2, and Zn(C104)2. On the other hand, an interaction with the perchlorate anion, probably consisting in its penetration into the second hydration sphere of the cation, takes place on increasing concentration in solutions of Cu(C104)2 and Mg(C104)2.