The standard (p degrees = 0.1 MPa) molar enthalpies of formation of crystalline 1,1,1-trifluoro-4-(2-thienyl)-4-hydroxy-3-buten-2-one, HTTFA, and 1,1,1-trifluoro-4-(2-thienyl)-4-mercapto-3-buten-2-one, HTTFAS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K were measured by microcalorimetry, with the following results : [GRAPHICS] The results demonstrate an increase in stabilization energy of approximate to (38.7 +/- 5.0) kJ.mol(-1) in HTTFA compared with thiophene and that in these molecules, the intramolecular hydrogen bond energy (O-H...O), appears approximate to 20 kJ.mol(-1) larger than for (S-H...O).