The crystal structure of the deuterated Tutton salt (ND4)(2)[Cu(D2O)(6)](SO4)(2) determined by X-ray diffraction at several temperatures between 100 and 321 K is reported. rile intermediate and longest Cu-O bond lengths of the Jahn-Teller distorted octahedral CU(D2O)(6)(2+) ion progressively converge as the temperature is raised above similar to 200 K, and this is accompanied by a partial rotation of the ammonium and sulfate groups. The g-values derived from the EPR spectrum of the compound exhibit similar behavior, as do those of similar to 1% Cu2+ doped into the isomorphous zinc(II) compound. The temperature dependence of the g-values of Cu2+-doped (ND4)(2)[Zn(D2O)(6)](SO4)(2) may be interpreted satisfactorily by a model which assumes a Boltzmann thermal distribution between two energy states which differ solely in the orientation of the Cu(D2O)(6)(2+) ion in the lattice, However; such a model does not satisfactorily explain the behavior of pure (ND4)(2)[Cu(D2O)(6)](SO4)(2), and it is suggested that this is due to cooperative interactions. A new model, in which the probable energy state of each complex is estimated after taking into account the likely orientations of its neighbors in the lattice, is described. Application of this model suggests that the thermal behavior is dominated by the cooperative interactions between complexes, these being transmitted via the hydrogen-bonding network. Comparisons are drawn with the cooperative interactions observed in more strongly coupled Jahn-Teller systems and in compounds for which the structural changes are associated with equilibria between two spin states.