Methyl and ethyl radicals have been produced on the surface of porous Vycor glass (PVG) at 77 K by UV photolysis of adsorbed azomethane and azoethane. respectively. The build-up of the radicals as a function of irradiation time has been studied by using the technique of electron spin resonance (ESR). Ethyl radicals are found to be slightly more difficult to generate than methyl radicals and both radicals were observed not to approach saturation within the period of irradiation employed. The variation of the rate of formation of the radicals with surface coverage and their formation during re-irradiation after previous decay for several hours at 77 K are discussed.