The fluorescence assay by gas expansion (FAGE) method for the measurement of the methyl peroxy radical (CH3O2) using the conversion of CH₃O₂ into methoxy radicals (CH₃O) by excess NO, followed by the detection of CH₃O, has been used to study the kinetics of the self-reaction of CH₃O₂. Fourier transform infrared (FTIR) spectroscopy has been employed to determine the products methanol and formaldehyde of the self-reaction. The kinetics and product studies were performed in the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) in the temperature range 268–344 K at 1000 mbar of air. The product measurements were used to determine the branching ratio of the reaction channel forming methoxy radicals, rCH₃O. A value of 0.34 ± 0.05 (errors at 2σ level) was determined for rCH3O at 295 K. The temperature dependence of rCH3O can be parametrized as rCH3O = 1/{1 + [exp(600 ± 85)/T]/(3.9 ± 1.1)}. An overall rate coefficient of the self-reaction of (2.0 ± 0.9) × 10^–13 cm³ molecule^–1 s^–1 at 295 K was obtained by the kinetic analysis of the observed second-order decays of CH₃O₂. The temperature dependence of the overall rate coefficient can be characterized by koverall = (9.1 ± 5.3) × 10^–14 × exp((252 ± 174)/T) cm³ molecule^–1 s^–1. The found values of koverall in the range 268–344 K are ∼40% lower than the values calculated using the recommendations of the Jet Propulsion Laboratory and IUPAC, which are based on the previous studies, all of them utilizing time-resolved UV–absorption spectroscopy to monitor CH₃O₂. A modeling study using a complex chemical mechanism to describe the reaction system showed that unaccounted secondary chemistry involving Cl species increased the values of koverall in the previous studies using flash photolysis to initiate the chemistry. The overestimation of the koverall values by the kinetic studies using molecular modulation to generate CH₃O₂ can be rationalized by a combination of underestimated optical absorbance of CH₃O₂ and unaccounted CH₃O₂ losses to the walls of the reaction cells employed.