The pulse radiolysis method has been used to generate methyl peroxy radicals in the presence of high concentrations of NO. The CH₃O₂ radicals are then rapidly converted to CH₃ONO by reaction with NO. Methyl nitrite absorbs strongly in the UV. Hence the initial absorption attributable to CH₃O₂ is rapidly replaced by absorption by CH₃ONO. By varying the monitoring wavelength one can readily determine the isosbestic point, where σ(CH₃O₂) = σ(CH₃ONO) + σ(NO₂). For methyl peroxy radicals the isosbestic point was determined to lie in the wavelength range 228-230 nm, and hence σ(CH₃O₂) = (3.9 ± 0.4) × 10^-18 cm² molecule^-1. Quoted errors are the sum of statistical (2σ) and our estimate of potential systematic errors (5%). As part of the present work rate constants for reactions 11 and 14 at 298 K and 600 mbar total pressure of SF6 were determined to be CH₃ + NO + M → CH₃NO + M (11) F + O₂ + M → FO₂ + M (14) k₁₁ = (1.07 ± 0.03) × 10^-11 cm³ molecule^-1 s^-1 and k₁₄ = (2.35 ± 0.20) × 10^-13 cm³ molecule^-1 s^-1 at 298 K. Results are discussed with respect to previous literature data.