On heating in toluene to 180° and on treatment with BF3.Et2O in CH2Cl2 at room temperature, 1,3-dienes react with the C=S group of 1,3-thiazol-5(4H)-thiones 1 in a reversible Diels-Alder reaction to give spiro[4.5]- heterocycles of type 6. A 1:1 mixture of two regioisomeric cycloadducts is formed in the thermal reaction with 2-methylbuta-1,3-diene (isoprene, 5b). In contrast, the formation of one regioisomer is strongly preferred in the BF3-catalyzed reaction. Frontier-orbital control as well as steric factors seem to be responsible for the observed regioselectivity. BF3-Catalyzed, cyclic 1,3-dienes and 1 also undergo a smooth Diels-Alder reaction. Whereas cyclohexa-1,3-diene (5c) reacts with 1a and 1b to give a single isomer (presumably the ‘eco’-adduct), cyclopenta-1,3-diene (5d) leads to a ca. 3:I mixture of ‘eco’-and ‘endo’-isomer.