Dipeptides were derivatized with phenyl thiocarbamyl (PITC) to sulfurcontaining compounds. The reaction was performed directly on the chromatographic plate in normal phase chromatography. The detection system was based on an iodine-azide reaction that was induced by sulfur compounds. The procedure involved spraying the developed plate with sodium azide and starch solution and, subsequently, exposing the plate to iodine vapor. Due to the catalytic effect of C5 5S bonds, white spots appeared on the violet-gray background. The method was found to be competitive with the other detection techniques and its sensitivity range was as low as pmol per spot.