The photodecomposition of a-methoxydeoxybenzoin (1) and a,a-dimethoxydeoxybenzoin (2) was investigated both in the absence and in the presence of 1,l-diphenylethylene. The resulting products indicate that benzoyl radicals add to the olefinic double bond (initiation of polymerization), whereas the a-methoxybenzyl radicals generated from 1 do not add to the olefinic double bond (no initiation), but preferentially combine with other radicals (termination of polymerization). This reaction is largely suppressed in the case of a,adimethoxybenzyl radicals generated from 2 owing to sequential fragmentation into inactive methylbenzoate and highly reactive methyl radicals which add to the olefinic double bond and thus contribute to the initiation. *) Part 2: H.