Three N-acyl-NЈ,NЈ-diethylthioureas, RCONHCSNEt 2 , R = i Pr, i Bu, t Bu, have been prepared and characterised. The standard ( p o = 0.1 MPa) molar enthalpies of combustion in oxygen of the three crystalline compounds, at T = 298.15 K, have been measured by rotating bomb-combustion calorimetry, and the standard molar enthalpies of sublimation of the compounds by microcalorimetry. These values were used to derive the standard molar enthalpies of formation of the compounds in their crystalline and gaseous phases, respectively. The derived standard molar enthalpies of formation in the gaseous state are discussed comparatively. Acid constants and some complex stabilities have been measured pH-potentiometrically in dioxane-water mixture. The crystal structure of N,N-diethyl-NЈ-isovaleroylthiourea is presented and shows a delocalization of the π electrons of the C᎐ ᎐ S group over the carbon-amine nitrogen bond, CS-NEt 2 , stabilising the molecule, in accordance with the thermochemical results.