The structure of PdSO₃(NH₃)₃ has been determined. It forms crystals of space group P2₁/c, with a= 7·84, b= 6·96, c= 13·77 Å, β= 126·7°, with four molecules in the unit cell. The structure was solved by Patterson methods, and refined by full-matrix least-squares. There is square planar co-ordination around the palladium atom, the sulphito-group being bonded through sulphur and keeping the shape of a trigonal pyramid. The Pd–S distance is 2·254 ± 0·006 Å, the average Pd–N distance is 2·11 ± 0·02 Å, the average S–O distance is 1·50 ± 0·02 Å, and the average O–S–O angle is 108·6°± 1·1°. These results indicate that there is a strong Pd–S bond with some π-character, and that, in agreement with the interpretation of the infrared spectra, the S–O bonds in the complexed sulphito-group are stronger than in the sulphite ion.