The objective of the study was to compare five methods with varying chemical approaches to the speciation of aluminum (Al). The 8-hydroxyquinoline (HQ) and ferron procedures were used to estimate the inorganic, monomeric forms of AL at reaction times of 15 and 30 s, respectively. Two other procedures, an ion exchange column procedure and a chelating resin procedure, were used primarily to measure organically bound forms of Al. These were compared with a fluoride (F) electrode technique which quantifies Al3+ activity using equilibrium thermodynamic calculations and measured values of F activity and total F. While the evidence presented is by no means exhaustive, when combined with the evidence of other investigators, it adds credence to the quantificatiion of Al3+ activity obtained using the fluoride-sensitive electrode and suitable thermodynamic calculations. Perhaps even more importantly, it provides a means of testing other procedures for their ability to measure the toxic forms of Al under well-controlled experimental conditions. (Copyright (c) Journal of Soil Science Society of America, 1987.)