1. In conjunction with triethylamine or pyridine alkene epoxides serve as catalysts for the trimerization of methyl isothiocyanate. 2. Methyl isothiocyanate trimer readily reacts with alkene epoxides in presence of TEAB with formation of the previously undescribed spiro compounds (I), (IV), (V), and (VI). When heated, the latter break down into alkene episulfides and trimethyldithio-s-triazine-2,4,6(1H,3H,5H)-trione. 3. The further heating of trimethyldithio-s-triazine-2,4,6(1H,3H,5H)-trione with ethylene oxide in presence of TEAB leads, via the formation of intermediately isolated spiro compounds, to trimethyl-s-triazine-2,4,6(1H,3H,5H)-trione. In this way all the sulfur atoms of trimethyl-s-triazine-2,4,6(1H,3H,5H)-trithione are replaced by oxygen atoms.