The complexes t- and p-[Co(tren)(NH₃){OP(H)(O)₂}]ClO₄ (1·ClO₄ and 2·ClO₄, respectively) and syn(OP(H)(O)₂), anti(OH₂)-[Co(cyclen)(OH₂){OP(H)(O)₂}]ClO₄ (3·ClO₄, cyclen = 1,4,7,10-tetraazacyclododecane) have been prepared. Single crystal structures have been determined for 1·ClO₄·2H₂O and 3·ClO₄·3H₂O. In weakly acidic aqueous solution 3 equilibrates with its syn(OH₂),anti(OP(H)(O)₂) isomer and the chelate [Co(cyclen){O₂P(H)O}]⁺. Kinetic studies of acid catalysed H/D exchange in H₂DPO₃ (4-D) and [Co(NH₃)₅{OP(D)(OH)O}]²⁺ (5-D) in water (55.0 °C, I = 1.0 mol dm^–3, NaClO₄) showed that 4-D (k_ex = (1.6 ± 0.2) × 10^–4 dm³ mol^–1 s^–1) is four-fold more reactive than 5-D (k_ex = (4.0 ± 0.5) × 10^–5 dm³ mol^–1 s^–1) and that exchange likely involves P(OH)₃ and [Co(NH₃)₅{OP(OH)₂}]²⁺ intermediates, respectively. Bromine oxidation of [Co(NH₃)₅{OP(H)(O)₂}]⁺ (5-H, 25 °C, pH 1.22–4.34, I = 1.0 mol dm^–3, NaClO₄) follows the rate equation k_obs = k^HK_a^H [5-H]_T/(K_a^H + [H⁺])(1 + K_Br3[Br^–]), where K_a^H ((4.2 ± 0.5) × 10^–4 mol dm^–3) corresponds to the independently measured acid dissociation constant of the 5-H conjugate acid, K_Br3 (17.6 dm³ mol^–1) = [Br₃^–]/[Br₂][Br^–], and where k^H (1316 ± 20 dm³ mol^–1 s^–1) represents the specific rate constant for reaction of Br₂ with 5-H. For 5-D as substrate k^D = 560 dm³ mol^–1 s^–1 (k_H/k_D = 2.4). Similar studies with H₃PO₃ (4-H, 25 °C, pH 1.27–2.55, I = 1.0 mol dm^–3, NaClO₄) showed that bromine oxidation follows the rate equation: k_obs = k^HK_a1^HK_a2^H[4-H]_T/([H⁺]² + K_a1^H[H⁺] + K_a1^HK_a2^H )(1 + K_Br3[Br^–]), with K_a1^H (0.108 mol dm^–3) and K_a2^H (1.67 × 10^–6 mol dm^–3) corresponding to the independently measured first and second acid dissociation constants of 4-H, and with k^H (2.1 × 10⁵ dm³ mol^–1 s^–1) representing the specific rate constant for reaction of Br₂ with HPO₃^2–. For 4-D as substrate k^D = 1.22 × 10⁵ dm³ mol^–1 s^–1 (k_H/k_D = 1.7). NMR studies showed that there is no loss of ¹⁷O label to solvent from H₃P¹⁷O₃, either during H/D exchange or on bromine oxidation. Mechanisms for the reactions are discussed.