AbstractGonadotropin releasing hormone (GnRH), D‐Phe6‐GnRH and thyrotropin releasing factor (TRF) were tritiated by direct catalytic exchange using Rh/Al2O3+ HT under conditions which lead in model deuterations of Nα‐acetylhistidine amide to a high incorporation of deuterium into position 5 of the histidine ring. Specific activities up to a range of 400 GBq/mmol in form of nonhydrolyzable tritium are attainable after removal of the label incorporated into position 2 of the histidine ring. A crucial reason for diminished specific activities was found to be a catalystmediated hydrogen transfer between the peptides and traces of water, contained in the reaction mixture, competing with the trit