The crystal structure of thiosulphatopenta-aminecobalt(III) chloride monohydrate, [Co(NH₃)₅S₂O₃]Cl, H₂O, has been determined by three-dimensional X-ray analysis from 738 non-zero reflexions measured at room temperature by photographic methods. The compound crystallizes in the orthorhombic system, space group Pnma with a= 13·12 ± 0·03, b= 7·97 ± 0·02, c= 10·29 ± 0·02 Å, Z= 4. The crystal structure has been solved by Patterson and Fourier methods and refined by least-squares calculations to R 0·082. The co-ordination around the metal atom is octahedral, with the thiosulphate group bonded to the cobalt atom through its terminal sulphur. Some important average distances are : Co–S 2·284, Co–N 1·964, and S–O 1·449 Å; the average O–S–O angle of the thiosulphate group is 111·6°. The sulphur–sulphur bond length in the thiosulphate group is found to be 2·057 Å, which is significantly longer than in the free ion.