Sulphinatoiron(III) porphyrins [FeL(SO₂R)](L = porphyrinate, R = alkyl or aryl) have been prepared by the insertion reaction of sulphur dioxide into the Fe–C σ bond of alkyl(or aryl)iron (III) porphyrins [FeLR]. The obtained products are easily oxidised by molecular oxygen to the corresponding sulphonate derivatives, also generated by acid hydrolysis of the Fe–O bonds of the µ-oxo-dimers [(FeL)₂O] by the appropriate sulphonic acid. The stereochemistry of the metal centre has been established by ¹H n.m.r., e.s.r., and i.r. measurements and confirmed by the crystal structure determination of the title compound by X-ray diffraction methods. The crystals are triclinic, space group P1, with a= 11.01 (2), b= 11.09(1), c= 11.37(2)Å, α= 107.2(1), β= 107.9(1), γ= 110.2(1)°, and Z= 1. The structure has been solved from diffractometer data by direct and Fourier methods and refined by full-matrix least-squares calculations to R= 0.097 for 1 901 reflections. The iron atom is five-co-ordinated by the four nitrogen atoms of the porphyrin and one oxygen atom of the sulphinato-group [Fe–O 1.92(1)Å].