The present study reports a differential scanning calorimetry (DSC) study of 2-thiobarbituric acid [CAS Registry No. 504-17-6] and its derivative 1,3-diethyl-2-thiobarbituric acid [CAS Registry No. 5217-47-0] in the temperature interval from T = 268 K to their respective melting temperatures. Temperatures, enthalpies, and entropies of fusion of both compounds are reported. 2-Thiobarbituric acid exhibits solid–solid phase transitions in the temperature interval studied. The polymorphic form of commercially available 2-thiobarbituric acid was identified by powder X-ray diffraction comparisons with the literature, and the heat capacity was measured from T = (268 to 430) K, temperatures below the first observable phase transition. The heat capacity of 1,3-diethyl-2-thiobarbituric acid was measured from T = (268 to 365) K. Commercially available 1,3-diethyl-2-thiobarbituric acid was likewise characterized by its X-ray powder diffraction pattern. Total phase change entropies and heat capacities of the two materials were also estimated by group additivity.