The Course of the reaction of trithiocyanuric acid (TTCA) with oxiranes, propylene oxide (PO), and epichlorohydrin in a dimethyl sulfoxide solution was monitored by the acid number and epoxy number. The involvement of one of the tautomers of TTCA was postulated on the basis of the studied reaction between TTCA and 3 equiv of oxirane. It was also evidenced that the use of a catalyst was necessary for the reaction between TTCA and an excess of PO. (c) 2006 Wiley Periodicals, Inc.