Thioxanthen-10-io- and 2,4-dimethylthioxanthen-10-io-(bismethoxycarbonyl)methanides and their 9-alkyl derivatives were synthesised by the reaction of the corresponding thioxanthens with bismethoxycarbonylcarbene generated thermally from dimethyl diazomalonate in the presence of copper(II) sulphate. The stereochemistry of the ylides was determined by examination of the n.m.r. spectra and by X-ray analysis