The products of the self-reaction of methylperoxy radicals have been determined at 296 K in a 140-L chamber using continuous ultraviolet photolysis with FTIR detection. The branching fraction for the reaction channel giving methoxy radicals is found to be (41 +/- 4)%, in good agreement with two earlier studies but somewhat higher than the most recent investigations. No evidence was found for the production of CH3OOCH3 (yield <6%). The addition of ozone led to changes in the rates of formation of the major products, which could largely be explained by chemistry of the OH radical. However, some evidence was found for a slow reaction of CH3O2 With O-3 occurring with a rate coefficient of 1 x 10(-17) cm(3) molecule(-1) s(-1), with an uncertainty of a factor of 2. As part of the present work a relative rate technique was used to measure k(Cl + CH3N2CH3) = (4.8 +/- 0.6) x 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The decay of peroxy radical concentrations in the nighttime clean troposphere is interpreted in terms of the product branching ratios and the rate coefficient for the reaction of CH3O2 + O-3.