Thermochemical properties for reactants, intermediates, products, and transition states important in the acetyl radical (CH3C*(=O)) + O-2 reaction system are analyzed with density functional and ab initio calculations, to evaluate reaction paths and kinetics in both oxidation and pyrolysis. Enthalpies of formation (DeltaH(f,298)(o)) are determined using isodesmic reaction analysis at the CBSQ composite and density functional levels. Entropies (S-298(o)) and heat capacities (C-p(o)(T)) are determined using geometric parameters and vibrational frequencies obtained at the HF/6-31G(d') level of theory. Internal rotor contributions are included in S and C-p(T) values. The acetyl radical adds to O-2 to form a CH3C(=O)OO* peroxy radical with a 35 kcal/mol well depth. The peroxy radical can undergo dissociation back to reactants, decompose to products, CH2C=O + HO2 via concerted HO2 elimination (E-a = 34.58 kcal/mol), or isomerize via hydrogen shift (E-a = 26.42) to form a (CH2C)-H-.(=O)OOH isomer. This (CH2C)-H-*(=O)OOH isomer can undergo beta scission to products, CH2C=O + HO2 (E-a = 31.41), decompose to a cyclic ketone, YCOC(=O) + OH via OH elimination (E-a = 19.97, Y cyclic), decompose to a diradical, (CH2CO)-H-.(O-*) + OH via simple RO-OH bond cleavage (E-a = 27.57), or isomerize back to the CH3C(=O)OO. isomer. Rate constants are estimated as function of pressure and temperature using quantum Rice-Ramsperger-Kassel analysis for k(E) and master equation for falloff. Important reaction products are stabilization of CH3C(=O)OO. peroxy adduct at low temperature and, at higher temperatures, formation of a diradical, (CH2CO)-H-.(O-*), + OH and CH2C=O + HO2 are dominant. DeltaH(f,298)(o) values are estimated for the following compounds at the CBSQ level: (kcal/mol) CH3C*(=O) (-3.08), (CH2CHO)-H-. (3.52), CH3C(=O)OOH (-84.80), CH3C(=O)OO* (-38.57), (CH2C)-H-.(=O)OOH (-32.95), and YCOC(=O) (-44.42). A mechanism for pyrolysis and oxidation of the acetyl radical is constructed. Reaction of acetyl with O-2 versus unimolecular decomposition is evaluated versus temperature and pressure. Related oxygen bonds in acetyl hydroperoxide are predicted to be stronger than corresponding bonds in alkyl hydroperoxide.