Partial molar volumesof the Fe2+, Ni2+, Fe3+, Cr3+, Rh3+, and M(NH3)5OH3+(M = Cr, Co, Rh, Ir) aqueous ions have been obtained by extrapolation from density measurements at 298.15 K on the perchlorate salt solutions containing added HClO4where necessary to suppress hydrolysis or oxidation. The values for these high-spin aqua-ions and others in the literature can be represented satisfactorily for ionic chargesz = 1 through 4 bywhereis the absolute partial molar volume in cm3mol−1of Mz+(aq) relative tofor H+(aq), andris the Shannon radius in pm of Mz+with coordination numbernc. Volumes of activation ΔVex* for water exchange correlate inversely withwithin the series M2+(aq) and M(NH3)5OH23+(aq) at least, allowing predictions of ΔVex* to be made fromor the above equation. The physical origin of these correlations is considered.