Thermochemical properties for important species in the formyl methyl radical ((CH2CHO)-H-.) + O-2 reaction system are analyzed to evaluate reaction paths and kinetics in both oxidation and pyrolysis. Enthalpies of formation (DeltaH(f298)degrees) are determined using isodesmic reaction analysis at the CBSQ composite and density functional levels. Entropies (S(298)degrees) and heat capacities [C(p)degrees(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 formyl methyl radical adds to O-2 to form a C(OO.)H2CHO peroxy radical with a 27.5 kcal/mol well depth. The peroxy radical can undergo dissociation back to reactants, decompose to CH2CO + HO2 via HO2 elimination, or isomerize via hydrogen shift to form a C(OOH)(H2CO)-O-.. This C(OOH)(H2CO)-O-. isomer can undergo beta scission to products, CH2CO + HO2, decompose to CO + CH2O + OH, or decompose to a diradical, (CH2OCO)-C-.-O-. + OH via simple RO-OH bond cleavage. Rate constants are estimated as a function of pressure and temperature using quantum Rice-Ramsperger-Kassel (QRRK) analysis for k(E) and master equation for falloff. Important reaction products are stabilization of the C(OO.)H2CHO peroxy adduct at low temperature and CO + CH2O + OH products via intramolecular H shift at high temperature. DeltaH(f298)degrees values are estimated for the following compounds at the CBSQ level: (CH2CHO)-H-. (3.52 kcal/mol), C(OOH)H2CHO (-56.19 kcal/mol), C(OO.)H2CHO (-21.01 kcal/mol), C(OOH)(H2CO)-O-. (-19.64 kcal/mol). A mechanism for pyrolysis and oxidation of the formyl methyl radical is constructed, and the reaction of the formyl methyl radical with O-2 versus unimolecular decomposition is evaluated.