Alkyl peroxides, trioxides, and the corresponding radicals are important in atmospheric chemistry, photochemical smog formation, and combustion processes, but accurate and widely accepted enthalpy data for these species are not available. In this work we verify enthalpy data for several compounds and derive the corresponding group values for use in the group additivity method. Isodesmic reactions and ab initio calculations (MP4SDTQ/6-31G*parallel to MP2/6-31G* and G2) are used to determine enthalpies of formation for the following compounds (in kcal mol(-1)): CH3OOH (-31.8), C2H5OOH (-39.9), iPrOOH (iPr = (CH3)(2-) CH-, -49.0), (CH3)(3)COOH (tBu = (CH3)(3)C-, -58.4), iPrOO . (-15.1), tBuCOO . (-25.2), CH3OOCH3 (-31.0), C2H5OOC2H5 (-47.2), iPrOOiPr, (-65.4), tBuOOtBu (-84.2), HOOOH (-23.0), CH3OOOH (-22.2), CH3OOOCH3 (-21.4). Our results on isodesmic reactions indicate that group additivity is an accurate method to estimate enthalpies (Delta H-f degrees(298)) Of alkyl peroxides and trioxides. Bond enthalpies are determined as follows: HOOO-H(82.6), CH3O2-H (86.6), C2H5O2-H (86.1), iPrO(2)-H (86.0), tBuO(2)-H (85.3), HOO-OH (35.8), CH3O-OCH3 (38.8), CH3OO-OH (34.2), CH3O-OOH (29.6), CH3O-OOCH3 (28.0), iPr-OO (36.6), tBu-OO (37.5). The recommended enthalpy group values of (O/C/O) and (O/O-2) are -5.5 and 9.6 kcal mol(-1), respectively.