Irradiation of substituted uracil derivatives with ultraviolet light causes changes in the infrared spectrum possibly associated with the formation of loosely associated species. Results are given for uracil, thymine, 5-iodouracil, 5-bromouracil, 1-ethyluracil, 5-mercaptouracil and 1-cyclohexyluracil. The latter three compounds do not exhibit evidence of new bond formation while the others show changes in band intensity and the appearance of new bands.