The atmospheric oxidation of methyl hydroperoxide by the hydroxyl radical has been investigated employing high level theoretical methods. This reaction is important in the chemistry of the troposphere because these species contribute to the oxidizing capacity of the atmosphere and therefore we have studied the bare reaction and the effect of the relative humidity as well. In both cases the reaction can proceed either by abstraction of the terminal hydrogen atom of the OH group, producing CH₃O₂ + H₂O, or by abstraction of one hydrogen atom of the CH₃ group, forming H₂CO + OH + H₂O. We have employed BH&HLYP, QCISD and CCSD(T) theoretical methods along with 6-311+G(2df,2p), aug-cc-pVTZ, aug-cc-pVQZ and CBS basis sets to investigate the reaction mechanism, and conventional and variational transition state theory to study the kinetics of the reaction. For the bare reaction we have computed at room temperature, a rate constant of 3.59 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for the formation of CH₃O₂ + H₂O and of 1.68 × 10⁻¹² cm³ molecule⁻¹ s⁻¹ for the production of H₂CO + OH + H₂O, with branching ratios of 68% and 32% respectively. Water vapor enhances the rate constant for the formation of CH₃O₂ + H₂O between 2 and 19%, depending on the temperature and relative humidity, whereas the rate constant for the production of H₂CO + OH + H₂O is enhanced between 0.3 and 5% by the effect of water vapor under the same conditions, which means that the branching ratio for the formation of CH₃O₂ + H₂O is increased up to 2.5%.