The heat capacities of 6-chlorouracil and 5-chlorouracil isomers were measured using a Quantum Design physical properties measurement system (PPMS) based on a heat pulse relaxation method over the temperature range from (1.9 to 300) K, and the experimental data were fitted as a function of temperature using a series of theoretical and empirical models for the appropriate temperature ranges. The results of these fits were used to calculate thermodynamic function values, C-P,m(0), Delta S-T(0)m(o), and Delta H-T(0)m(o), from T= (0.5 to 300) K. The standard molar heat capacities, entropies and enthalpies of 6-chlorouracil and 5-chlorouracil at 298.15 K and 0.1 MPa were determined to be C-p,m(o) = (136.94 +/- 1.37) J K-1 mol(-1) and (137.95 +/- 1.38) J K-1 mol(-1), S-m(o), = (156.87 +/- 1.57) J K-1 mol(-1) and (154.97 +/- 1.55) J K-1 mol(-1) and H-m(o) = (23.01 +/- 0.23) kJ mol(-1) and (23.07 +/- 0.23) kJ mol(-1), respectively. (C) 2016 Elsevier B.V. All rights reserved.