During sulfur vulcanization accelerated by guanidines (OTBG, DPG, DOTG) isothiocyanates (ITC) are formed inevitably. ITC are obtained both by splitting of thiourea originated from guanidines and by reaction of primary amines with CS, (COS) donating compounds. The mechanism of the ITC formation was studied on unfilled IR compounds vulcanized at 180 and 210 degrees C. The reaction products were analysed by GC/MS and HPLC/MS. Increasing sulfur dosage and higher vulcanization temperature promote the emission of ITC, Among the guanidines the highest amount of ITC is formed with OTBG. By addition of thiurams to the guanidine mixtures the amount of ITC increases frequently. Against that, MBT as secondary accelerator causes a retarding effect. Especially with OTBG/MBT-mixtures the lowest amounts of ITC are reached, Therefore small changes to either formulation or processing conditions can significantly reduce ITC in fumes.