The molar heat capacities at constant pressure of thiourea (TU), 1-methyl-2-thiourea (MMTU), 1-ethyl-2-thiourea (METU), 1,3-diethyl-thiourea (DETU) and 1,1,3,3-tetramethyl-2-thiourea (TMTU) were determined by differential scanning calorimetry (DSC) from 310 K to about (10 to 30) K below their melting points. Values at 298.15 K were obtained by extrapolating the smoothed fitting equations as a function of temperature. For 1,3-dimethyl-2-thiourea (DMTU), measurements at around room temperature were used for direct determination of the C-p,C-m experimental value at 298.15 K. C-p,C-m’s at 298.15 K of all compounds increased linearly with the number of carbon atoms in the alkyl substituents. The contribution of (25.5 +/- 1.5) J K-1 mol(-1) per-CH2-group was in good accordance with the values previously obtained for some series of small peptides. The molar heat capacities of isomeric METU and DMTU were almost identical, whereas those of isomeric DETU and TMTU displayed an appreciable divergence.