The nucleoside transport inhibitor 6-[(4-nitrobenzyl)thio]-9-(β-d-ribofuranosyl)purine, NBMPR, has been used successfully in photoaffinity labeling. We have studied the mechanism for photocleavage of the benzyl-sulfur bond by using substituted benzyl phenyl sulfides as analogues of NBMPR. This has enabled us to enhance the photoreactivity of the benzyl-sulfur bond. We have also performed “radical clock” studies with a hexenyl side chain to trap reactive intermediates. The mechanistic interpretation from the substituent and side chain studies is that the benzyl-sulfur moiety is photocleaved via a homolytic pathway.