The standard massic energies of combustion of three substituted bicyclic compounds : (4R*,4’R*,5R*,5’R*)-5,5’-diphenyl-3,3’,4,4’-tetramethyl-2,2’-bioxazolidine; (2R*,3R*,6R*,7R*)-2,6-diphenyl-3,4,7,8-tetramethyl-cis-perhydro-[1,4]-oxazino-[3,2-b]-[1,4]-oxazine; and (2R*,3S*,6R*,7S*)-2,6-diphenyl-3,4,7,8-tetramethyl-cis-perhydro-[1,4]-oxazino-[3,2-b]-[1,4]-oxazine (these compounds will be referred as C1, C2, and C3, respectively, and their formulae are given in figure 1) were determined by oxygen bomb combustion calorimetry. The enthalpies of sublimation were determined by the Knudsen effusion method using a quartz-crystal microbalance. Based on the experimental results, the standard molar enthalpies of formation of the solid and gaseous compounds were determined. A summary of experimental and derived results at the temperature T = 298.15 K is shown below. [GRAPHICS] From these results it is concluded that the destabilization energy of C3 in relation to C2, due to the extra steric repulsion of the cis-methyl and phenyl substituents in C3 is approximately 12 kJ . mol(-1) in the condensed phase and is increased to approximately 19 kJ . mol(-1) in the gaseous phase. It was also verified that formation of oxazino-oxazines over bioxazolidine is thermodynamically favored, because the enthalpy of formation of the latter is less negative.