The hydrolysis Of UO22+ was studied potentiometrically (H+-glass electrode) in NaCl (0.1-4.5 mol L-1) and NaNO3 (0.1-1 mol L-1) aqueous solution, at pH < 6 and t = 25 degreesC. Computer calculations indicate the presence of the species UO2(OH)(+), (UO2)(2)(OH)(2)(2+), (UO2)(3)(OH)(4)(2+), (UO2)(3)(OH)(5)(+), and (UO2)(3)(OH)(7)(-) whose equilibrium constants are reported together with parameters for dependence on ionic strength for both ionic media. Extrapolation to infinite dilution gave (beta(pq) = [(UO2)(p)(OH)(q)((2p-q))][H+](q)[UO22+](-p)): log beta(11) = -5.19 +/- 0.05, logbeta(22) = -5.76 +/- 0.04, log beta(34) = -11.82 +/- 0.08, log beta(35) = -15.89 +/- 0.06, and log beta(37) = -29.26 +/- 0.10. The speciation scheme was critically examined in the light of uncertainties derived by error propagation. The significance of the species found is discussed in relation to the different experimental conditions (concentration of UO22+ and medium). A comparison with literature findings is made.