Alkyl peroxyls form in the atmospheric oxidation of hydrocarbons and in their combustion. When NO concentration is low, they can appreciably react with themselves. This reaction has both propagation and termination channels. Multireference second-order perturbative energy calculations CAS(16,12)-PT2/6-311G(2df,p) have been carried out on the CAS(8,8)-MCSCF/6-311G(d,p) geometries pertaining to the reaction pathways explored. The tetroxide intermediate put forward first by Russell in 1957 is found as a stable energy minimum, but the calculations indicate that, as the system moves from atmospheric to combustion temperatures, its formation becomes problematic. A concerted synchronous transition structure, apt to connect it with the termination products, formaldehyde, methanol, and dioxygen, is not found. The concerted dissociation of the two external O-O bonds in the tetroxide leads to the (3)((CH3O.)-C-2)(2)...O-3(2) complex, with overall singlet spin multiplicity. Both termination via H transfer, to give H2CO, CH3OH, and O-2, or dissociation to 2 CH3O. + O-2 (possible propagation) are feasible. The former could occur in principle with production of either excited O-1(2) or excited (H2CO)-H-3. However, if a sufficiently easy intersystem crossing (ISC) could take place in the complex, the process would end up with all ground-state molecules. The (possible) propagation channels are favored by higher temperatures, while lower temperatures favor the ISC mediated termination channel. A fairly good qualitative agreement with experimental T dependence of the relevant branching ratio is found. From the tetroxide over again, dissociation of a single external O-O bond leads to CH3O. and CH3O3., or possibly to a (1)(CH3O....CH3O3.) complex, but further transformations along this line are not competitive. (C) 2003 American Institute of Physics.