Abstract Few analytical techniques are sensitive enough to detect environmental concentrations of uranium in waters where humic acids are present. One objective of this study was to devise a technique for measuring humic-complexed and uncomplexed uranium at these concentrations. Three techniques were combined to determine the binding capacity and conditional stability constant of uranium with Aldrich ® humic acid. Free UO 2 2+ was separated from bound by chelating resin; the CC bonds were destroyed by photo-oxidation and UO 2 2+ was quantified by laser fluorometry. The binding capacity (BC) of 3·5 mg C liter −1 Aldrich humic acid solution was estimated to be 1·14 × 10 −6 M UO 2 2+ with an asymptotic standard error of 5·0 × 10 −8 M UO 2 2+. UO 2 2+ was bound to humic acid by a continuum of sites with different strengths. The frequency distribution of these sites was log-normal. A Gaussian-Scatchard model was used to estimate the overall conditional stability constant for uranium concentrations of 5·25 × 10 −8 M to 2·1 × 10 −7 M in the presence of 3·5 mg liter −1 humate (1·14 × 10 −6 M = φ). The estimates of the mean and standard deviation for the log of the stability constants were 6·5 and 0·8, respectively. When these mean and standard deviations were used to determine the mole-average number of binding sites at three points on the Gaussian distribution, the estimators of log-stability constants were found to be: K 1 = 5·2, K 2 = 6·5 and K 3 = 7·7 with mole fractions of the total number of binding sites associated with each region of 0·21, 0·55 and 0·21, respectively. The thermodynamic, geochemical simulation model GEOCHEM and the three-component Gaussian-Scatchard estimates allowed accurate prediction of the relative proportion of UO 2 2+ bound to humates for a soft water pond across the entire range of metal-ligand ratios studied. Approximately 22% of the UO 2 2+ was predicted to be associated with dissolved organic carbon.