[en] The rates of formation and decomposition reactions of di-μ-hydroxo-bis[uranyl(VI)] ion, (UO₂)₂(OH)₂²⁺, have been measured in aqueous nitrate solutions (ionic strength = 0.5 M; M = mol dm^-3) by ¹⁷O NMR spectroscopy. In weakly acidic solutions the reactions obey the rate law: d[(UO₂)₂(OH)₂²⁺]/dt = k₂[UO₂²⁺][UO₂OH⁺] - k_-2K_a[H⁺][(UO₂)₂(OH)₂²⁺]/2, where K_a = [(UO₂)₂OH³⁺]/[H⁺][(UO₂)₂(OH)₂²⁺]. A mechanism involving a single-bridged binuclear intermediate, (UO₂)₂OH³⁺, is proposed. Rate constants (M^-1 s^-1) at 25 deg C and activation parameters ΔH^≠ (kJ mol^-1) and ΔS^≠ (J K^-1 mol^-1) for the formation and decomposition are as follows: 1.52 x 10⁵, 18.9±1.5, and -82.4±5.0 for k₂ and 2.10 x 10⁵, 23.6±1.3, and -64.0±4.2 for k_-2K_a. The rate of the decomposition reaction is compared with that in several related systems. (author)