Ideal gas thermodynamic properties (Delta H-f degrees(298), S degrees(298), and C-p(T), 300 less than or equal to T/K less than or equal to 1500) for 34 cyclic oxygenated hydrocarbons are calculated using the PM3 method, including 12 species on which data are not previously reported. Enthalpies of formation obtained using PM3 are further corrected by -1.642 + 0.882 Delta H-f degrees(298,PM3), which is obtained by comparison to experimentally-determined Delta H-f degrees(298) of 10 cyclic oxygenated hydrocarbon molecules. Enthalpies of formation (Delta H-f degrees(298), in kcal mol(-1)) and entropy (S degrees(298), in cal mol(-1) K-1) for 12 species are calculated as follows : -57.24 and 64.10 for 1,3-dioxetane, 25.08 and 61.36 for dioxirene, -4.62 and 68.16 for 1,2-dioxolene, -49.03 and 66.11 for 1,3-dioxolene, 16.72 and 68.21 for 1,2,3-trioxolene, -32.96 and 73.79 for 1,2-dioxane, -11.45 and 75.07 for 3,4-dihydro-1,2-dioxin, -57.44 and 72.62 for 2,4-dihydro-1,3-dioxin, -59.82 and 73.30 for 1,2,4-trioxane, -13.05 and 74.80 for 1,2,3-trioxane, -38.08 and 74.68 for 1,2,4-trioxene, and 9.14 and 74.68 for 1,2,3-trioxane, respectively. Delta H-f degrees(298) of seven molecules is different from previously estimated values by more than 4 kcal mol(-1). Our recommended Delta H-f degrees(298) values (in kcal mol(-1)) are : dioxirane (-2.47), 1,2-dioxetane (-6.38), 2,3-dihydrofuran (-23.52), 2,5-dihydrofuran (-20.90), 1,2-dioxolane (-27.42), 1,2,4-trioxolane (-59.12), and 3,6-dihydro-1,2-dioxin (-9.80). The standard deviations of Delta H-f degrees(298) and S degrees(298) values determined in the present study are evaluated as 2.89 kcal/mol and 1.15 cal mol(-1) K-1. Group Additivity ring corrections for corresponding heterocyclics are derived.