The toxicity of A1 to aquatic biota varies with speciation. Driscoll (1980) has developed a scheme for determining three A1 fractions using the ferron method (Smith, 1971) to detect A1. We have compared the results obtained with Driscoll's method with those obtained when pyrocatechol violet is the complexing agent. The agreement is generally satisfactory. The latter method is more sensitive and less influenced by interferences. Results for water samples, mainly from two streams in southern Norway, are discussed. For most samples the labile monomeric Al-fraction dominates, the major species being either A13 + or AIF 2 +. Possible equilibria between A13 + and some minerals and the temperature changes in these equilibria, are discussed.Water, Air, and Soil Pollution 23 (1984) 81-95. 0049-6979/84/0231-0081502,25.