We present the first measurement of the thermal desorption kinetics of methyl radicals. Neutral methyl radicals are photochemically produced within a methyl iodide film condensed on MgO(100). A substantial fraction become trapped through inelastic collisions with the film and thermalize to the film temperature prior to desorption. Novel residence time measurements are made by comparing neutral and ion flight times in a time-of-flight spectrometer, tin Arrhenius analysis of methyl radical residence times provides a desorption barrier of 2.5 +/- 0.5 kcal mol(-1) and a pre-factor of 10(10+/-1) s(-1).