The initiation steps in the copolymerization of styrene and acrylonitrile with t-butoxyl radicals have been investigated using the nitroxide trapping technique. t-Butoxyl radicals add to styrene Eve times more rapidly than to acrylonitrile whereas methyl radicals, formed by beta-fragmentation of t-butoxyl radicals, add to acrylonitrile about four times faster than to styrene. Acrylonitrile also reacts fast with first generation styryl radical end groups. Trapped products containing both monomers can only be observed, however, by operating at very low nitroxide trap concentrations. This has been achieved by using a syringe pump to feed nitroxide trap to the reaction mixture at a controlled rate. Some information on rate constants for second monomer addition has been obtained. Solvent effects on the relative rates of addition of t-butoxyl radicals have been observed and discussed.