DSC measurements were carried out for [Ni(H2O)6](ClO4)2 (sampleH) and [Ni(D2O)6](ClO4)2 (sampleD) in the temperature range 300–380 K. For both compounds two anomalies on the DSC curves were detected. The results for sampleH are compared to those previously obtained using adiabatic calorimetry method. For both compounds studied in this work the
high-temperature transition appears at the same temperature while the low-temperature one is shifted towards higher temperatures
in sampleD. Disorder connected with H2O or D2O groups is suggested in the intermediate phase between the low- and high-temperature transitions.