Heat capacities of 2,2-dimethyl-1,3-propanediol (CH3)(2)C(CH2OH)(2) were measured in the temperature range between T = 13 K and T = 350 K using an adiabatic calorimeter. The compound underwent a first-order phase transition at T = (314.5 +/-0.1) K. The enthalpy and the entropy of transition were (12.52 +/-0.02) kJ.mol(-1) and (39.81 +/-0.08) J.K-1.mol(-1). respectively. Measurement of the fusion peak by d.s.c. showed that the purity of the sample was 0.9999 mass fraction and the entropy of fusion was 9.9 J.K-1.mol(-1). Another first-order phase transition was observed at T = (60.4 +/-0.1) K with the associated entropy change of (2.93 +/-0.05) J.K-1.mol(-1). Heat capacities of two deuterated samples, (CH3)(2)C(CH2OD)(2) and (CD3)(2)C(CD2OD)(2), were also measured and the results were compared with those on the natural compound. Possible mechanisms of the transition have been discussed from the isotope effects on the thermodynamic quantities associated with the transition. Standard thermodynamic functions of the compounds are tabulated.