A new monoclinic form of the title compound, C(14)H(17)N(3)O(3)S, has been found upon slow crystallization from water. Another monoclinic form of the compound was obtained previously from a mixture of dichloro-methane and diethyl ether [Clerici et al. (2002 ▶). Tetra-hedron, 58, 5173-5178]. Both phases crystallize in space group P2(1)/n with one mol-ecule in the asymmetric unit. The formally single exocyclic C-N bond that connects the -NEt(2) unit with the thia-zete ring is considerably shorter than the adjacent, formally double, endocyclic C=N bond. This is likely to be due to the extended conjugated system between the electron-donor diethyl-ammine fragment and the electron-withdrawing sulfonyl group. In the newly discovered polymorph, the meth-oxy group is rotated by almost 180° around the phen-yl-OCH(3) bond, resulting in a different mol-ecular conformation.