The compounds cis-5a,6,11a,12-tetrahydro[1,4]benzothiazino[3,2-b][1,4]benzothiazine(BTB) and 5a,6,11a,12-tetrahydro[1,4]benzoxazino[3,2-b][1,4]benzoxazine(BOB) have been studied by oxygen bomb combustion calorimetry and 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 a group-contribution scheme and from the experimental results, the group-contribution terms to the standard molar enthalpies of formation of the gaseous compounds for the molecular groups {C-(S)(N)(C)(H)} and {C(O)(N)(C)(H)} were calculated as -76.1 kJ.mol(-1) and -42.7 kJ.mol(-1), respectively. It is concluded that the standard molar enthalpy of formation in the gas phase is decreased by (340.4 +/- 9.0) kJ.mol(-1) when two C-b-S bonds and two CS bonds are substituted by two C-b-O bonds and two C-O bonds in the compound BTB to obtain BOB; here C-b refers to a carbon atom in the benzene ring.