Abstract The Riemschneider reaction of 3-thiocyanatoquinoline-2, 4 (1H, 3H)-diones with conc. H 2 SO 4 was investigated. Using different reaction conditions, 13 types of reaction products were isolated. Compounds bearing a Me, Et, or Bu group at C (3) afforded mainly [1, 3] thiazolo [5, 4-c] quinoline-2, 4-diones and 1, 9b-dihydro-9b-hydroxythiazolo [5, 4-c] quinoline-2, 4-diones. In the case of the 3-Bu derivatives of the starting compounds, C- ...