The equilibrium between [Ce(H2O)9]3+ and [Ce(H2O)8]3+ has been followed in aqueous solution at 298 K by variable‐pressure UV spectroscopy at 295 nm. The dervied volume of reaction for the dissociation of this enneaaqua ion is ΔV 0 = +10.9 cm3. mol−1. This value, together with the previously determined activation volume, ΔV ≠ = −6 cm3. mol−1, for H2O exchange on [Ln(H2O)8]3+ (Ln = Tb to Tm), allows the assignment of an associative interchange I a mechanism on these octaaqua ions.