Maleimides (2a−j) react with thiourea in refluxing ethanol to yield the corresponding N-aryl α-(2,3,4,5-tetrahydro-4-oxo-l,3-thiazole-5-yl) acetamides (6a−j) in 72–85%. The spectral and analytical data are consistent with the assigned structures for6a−j. Under similar conditions, isomalemides (3a−b and3d−f) furnish the corresponding6a−b and6d−f in 76–86%. While maleanilic acid (4a) itself failed to react with thiourea,4b−ē,4i and4j yielded a mixture of the corresponding6b−ē,6i and6j in 2–50% and fumaranilic acids (7b−ē,7i and7j) in 50–80%. Methyl maleanilates (5b,5d−f and5i) gave6b,6d−f and6i in 78–85%. The reaction course of Raney nickel desulphurisation of6a−b,6d,6f and mass spectral fragmentation of6b support the assigned structures.