A new method for measurement of the methyl peroxy (CH₃O₂ ) radical has been developed using the conversion of CH₃O₂ into CH₃O by excess NO with subsequent detection of CH₃O by fluorescence assay by gas expansion (FAGE) with laser excitation at ca. 298 nm. The method can also directly detect CH₃O, when no nitric oxide is added. Laboratory calibrations were performed to characterise the FAGE instrument sensitivity using the conventional radical source employed in OH calibration with conversion of a known concentration of OH into CH₃O₂ via reaction with CH₄ / O₂. Detection limits of 3.8 × 10⁸ molecule cm−³ and 3.0 × 10⁸ molecule cm−³ were determined for CH₃O₂ and CH₃O, respectively for a signal-to-noise ratio of 2 and 5 min averaging time. Averaging over 1 hour reduces the detection limit for CH₃O₂ to 1.1 × 10⁸ molecule cm−³ comparable to atmospheric concentrations. The kinetics of the second–order decay of CH₃O₂ via its self–reaction were observed in HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) at 295 K and 1 bar and used as an alternative method of calibration to obtain a calibration constant with overlapping error limits at the 1σ level with the result of the conventional method of calibration. The overall uncertainties of the two methods of calibrations are similar: 15 % for the kinetic method and 17 % for the conventional method and are discussed in detail. The capability to quantitatively measure CH₃O in chamber experiments is demonstrated via observation in HIRAC of CH₃O formed as a product of the CH₃O₂ self–reaction
