The structure of monochlorobis(diethyldithiocarbamato)iron(III) has been determined by X-ray diffraction methods. Refinement was carried out by the method of least squares to a final R of 0·15 for 1400 independent reflections. The compound crystallises in the monoclinic space group P21/c with Z= 4 in a unit cell with dimension a= 16·43 ± 0·01, b= 9·42 ± 0·01, c= 12·85 ± 0·01 Å, and β= 120·5 ± 0·3°. The configuration about the iron atom is approximately square pyramidal; the iron atom lies 0·62 Å above a plane of four sulphur atoms of the two ligand entities and is surmounted by a chlorine atom. Evidence is provided for extensive conjugation through the ligand, postulated elsewhere from i.r. spectroscopy studies.