A method is detailed for the study of the reaction of 2,4-dinitrophenylhydrazine with polyethylene films containing carbonyl groups. The difference between films u.v.-irradiated in air and those made with a ketone-containing copolymer is studied. There is also a study of the effects of thickness of film. Progress of reaction was monitored by u.v.-spectroscopy. The u.v. absorbance increased with time of immersion in the 2,4-dinitrophenylhydrazine solution, and with increasing thickness of film. An explanation is advanced for the difference between the two types of film: it is related to differences between the distribution of carbonyl groups through the films. Most of the carbonyl groups in the photo-oxidised films made from polyethylene were on the surface. There is evidence that 2,4-dinitrophenylhydrazine can penetrate and react throughout the films.