The pyrrolo [1, 2-c] thiazole generated by dehydration of dimethyl 5-methyl-1H, 3H-pyrrolo [1, 2-c] thiazole-6, 7-dicarboxylate 2-oxide acts as a thiocarbonyl ylide in its cycloaddition to electron deficient alkenes but as an azomethine ylide with electron deficient alkynes. The analogous pyrazolo [1, 5-c] thiazole, generated similarly, acts as a thiocarbonyl ylide with both types of dipolarophile. This behaviour is partially explained by Frontier MO theory.