Studies of the photochemical decomposition of carbonyl compounds have shown the importance of investigations on homologous compounds as evidence for the mechanisms of the primary photochemical processes in the individual substances. It seemed of interest, therefore, to undertake a similar study of a number of hydrocarbons. With these somewhat more stable compounds, it is important to be able to extend the investigations beyond the wave-length limits arbitrarily imposed by the use of quartz apparatus to include the shorter wave-lengths transmitted by fluorite. For this purpose a special apparatus has been developed. In the interpretation of results a knowledge of the absorption spectrum of the hydrocarbon concerned is of great assistance, and it seemed advisable, therefore, in the first instance to concentrate on the simpler hydrocarbons, the spectra of which had been measured. In the case of ethylene, which is the subject of the investigation reported here, such measurements have been made by Price and by Hilgendorff.