The temperature of the structural phase transition in cadmium calcium acetate hexahydrate as a function of Cu2+ concentration has been studied with electron paramagnetic resonance (EPR). The phase transition temperature was found to vary from 144 to 124 K as the amount of copper varied from 0.07% to 1.3% of the sample weight. An extrapolation of this data predicts a structural phase transition at 145 K in pure cadmium calcium acetate hexahydrate. Good fits of the data to second-order phase transition theory was obtained with critical exponents close to 0.30.