2,5-dihydrothiazoles are prepared by reacting aza-alkylene-dienes with sulfur at relatively low temperatures for example below about 160.degree. C. Where the reaction is carried out at relatively higher temperature, for example above about 160.degree. C., such as 160.degree. to 250.degree. C., 4,5-dihydrothiazoles are formed. In addition 2,5-dihydrothiazoles can be converted to 4,5-dihydrothiazoles by heating them to temperatures above about 160.degree. C.