The effect of 5 organic anions (citrate, malate, malonate, oxalate, and tartrate) on the concentration of Al measured with 2 colorimetric reagents [pyrocatechol violet (PCV) and aluminon] was determined as a function of reaction time. The results are compared with concentrations of reactive Al [Al3+ + AlOH2+ + Al(OH)2+ ] predicted from stability constants. The PCV reagent was found to overestimate this fraction, i.e. to underestimate the extent of complexation by oxalate, malonate, and malate, even when the shortest reaction time of 30 s was applied. However, the aluminon reagent provided a good estimate of ‘reactive’ Al at the 30 s reaction time in the presence of all organic anions studied. At a citrate and tartrate to Al ratio of 1 : 2, the ‘reactive’ Al concentration, determined from stability constant data, underestimated the extent of Al complexation. These deviations could be modelled by the addition of a further complex, with the composition Al2 (OH)L2+ for citrate and Al2(OH)2L2+ for tartrate. Tentative equilibrium constants for these species are presented.