The u.v. photolysis of statistical methyl methacrylate-methyl vinyl ketone copolymers containing relatively small amounts of methyl vinyl ketone is shown, by n.m.r. and g.p.c. analysis, to give rise to oligo(methyl methacrylate)s with allylic end-groups that are not readily polymerizable. Thus it is concluded that the photolysis proceeds predominantly via a Norrish type I process rather than by a modified Norrish type II process as previously suggested. The Norrish type II process, however, is a minor contributor to the overall reaction and gives rise to some oligomeric fragments with acrylic end-groups.