The anation of (α, β)S-(tetren)CoOH2+ by SO32− in alkaline medium (pH>12) produced the S-bonded sulphito complex exclusively. The reaction is reversible andkobs=(k1+k2[OH−])[SO32−]T+k−1[OH−] is obeyed at [SO32−]=0.005–0.006, [OH−]T=0.02−0.30 mol dm−3 (30–45°C,I=1.0 mol dm−3). At 35°C,k1=(3.6±0.6)×10−3 dm3 mol−1 s−1,ΔH≠=55±10 kJ mol−1,ΔS≠=−112±34 JK−1 mol−1,k2=(4.3±0.6)×10−2dm6 mol−2 s−1,ΔH≠=44±11 kJ mol−1,ΔS≠=−130±36 JK−1 mol−1. The unusually high rate of sulphite substitution particularly in the OH− independent path is attributed to the involvement of a reactive internal conjugate base due to the coordinated OH− assisted NH-deprotonation of the tetren moiety (Co(tetren)OH2+⇋Co(tetren-H)OH22+). Substantially low values of the activation parameters (ΔH≠ andΔS≠) particularly the activation entropy is consistent with associative interchange mechanism (Ia) which is further supported by the rate retardation by anionic micelles of SDS. For the pseudo-phase ion-exchange equilibrium (tetren) in the reaction CoOHW2++2NaM+⇋(tetren)CoOHM2++2NaW+, the calculated value ofK′ex is 219±60 at 35°C.