The compounds (4R, 5R)-3,4-dimethyl-5-phenyl-1,3-oxazolidine (OXA1), (4S,5R)-3,4dimethyl-5-phenyl-1,3-oxazolidine (OXA2) [Chemical Abstracts registry number 123618-06-4], and (4R,5R)-2,2,3,4-tetramethyl-5-phenyl-oxazolidine (OXA3) [Chemical Abstracts registry number 141271-51-4] have been studied by oxygen bomb combustion calorimetry, and the vapor pressures were measured at different temperatures by a static method. On the basis of the experimental results, the standard molar enthalpies of formation of the liquid and gaseous compounds were determined. A summary of experimental and derived results at the temperature T = 298.15 K is shown below; where Delta(c)U(m)degrees(l), Delta(c)H(m)degrees(l), Delta(f)H(m)degrees(l), Delta(l)(g)H(m)degrees(l), and Delta(f)H(m)degrees(g) denote the standard molar energy of combustion, the standard molar enthalpy of combustion, the standard molar enthalpy of formation of the liquid compound, the standard molar enthalpy of vaporization, and the standard molar enthalpy of formation of the gaseous compound, respectively.The standard molar enthalpy of formation in the gaseous phase is decreased by 7.8 kJ . mol(-1) from compound OXA2 to OXA1. This quantity is attributed to the different configuration of the methyl group at the C-4 position of these compounds. In the envelope conformation, OXA2 has a nearly gauche interaction between the phenyl and methyl substituents, while OXA1 has a nearly anti-interaction between the same substituents. In addition, with the experimental results for compounds OXA1 and OXA3, the group contribution terms to the standard molar enthalpies of formation of the gaseous compounds for the molecular groups {C-(O)(N)(H)(2)} and {C-(O)(N)(C)(2)} were derived as -104.5 kJ . mol(-1) and -118.8 kJ . mol(-1), respectively.