Abstract Stable products from the photolysis of HI in methyl-3 pentane (3 MP) or dimethyl-2, 3 butane (23 DMB) quenched solid at 77 K are analyzed by capillary gas chromatography. Selective formation of a tertiary radical is proposed for the 23 DMB/HI system; in 3 MP/HI, CH bond rupture is also localized at the weakest tertiary bond, however the scission is not selective.