The photochemical formation is described of N-arylacetyl lactams from the corresponding N-phenylglyoxylic acid derivatives of pyrrolidine and piperidine-2-carboxylic acids in an acetone:water mixture containing potassium carbonate. Evidence is presented for the possible formation and cleavage of a cyclic ether during this process which leads to the 1, 4-transfer of a carbonyl group.