The reaction of (αβ)-(tetren)CoOH2+ with S2O32- in the 7.25-8.28 pH range at 20-40°C yielded S- (yellow) and O- (purple) bonded thiosulfato(tetren)cobalt(III) complexes, the former in larger quantities. The rate determining step is preceded by diffusion-controlled ion-pair [(tetren)CoOH2+,S2O32-] formation. Replacement of coordinated OH- by S2O3-2 is interpreted in terms of an internal conjugate base mechanism: (tetren)CoOH2+ ⇌ (tetren-H)CoOH22+, the reactive amido conjugate base being generated by intramolecular proton transfer from the coordinated NH site. In acid medium the S-bonded (tetren)Co(S2O3)+ is highly stable to redox decomposition, in contrast to its pentaammine analogue. The complex however, undergoes base hydrolysis yielding the corresponding hydroxo complex. The rate and activation parameters for the base hydrolysis have been reported. Photolysis of O- and S-bonded isomers of [(tetrcn)CoS2O3]+ in acidic medium at 254 and 313nm, respectively, yielded aquation products accompanied by some decomposition of S2O22-. © 1996 Chapman & Hall.