The crystal and molecular structure of thiolphthalide has been determined by X-ray crystallographic analysis. The crystals of this compound are orthorhombic (space group P 2 1 2 1 2 1 ) with cell dimensions of a = 4.104 A., b = 11.835 A., and c = 14.602 A. The structure was determined by Patterson and Fourier methods and refined by least squares to a final R value of 0.070. The two C─S bond lengths are 1.83 A. and 1.77 A., with the latter bond adjacent to the carbonyl group. The angle subtended by the C─S bonds is 94°.