The amounts and forms of Al in soil solution are key factors in determining Al toxicity to plants. The Al species primarily responsible for phytotoxicity are free Al3+, AlOH2+ and Al(OH)2+, with free Al being the most significant. The soil properties that control the concentration of Al species in soil solution are poorly understood and are the focus of this investigation. Extractable Al was measured using 0.01M CaCl2 and the pyrocatechol violet method (Dougan and Wilson, 1974) on 193 top and sub-soil samples from south western-Australia. At pH (CaCl2) values less than about 4.5, Al becomes toxic to wheat roots but the pH at which toxicity occurs differs between soils and the causes of this variability are not well understood. For these Australian soils, extractable Al concentrations at pH values between 3.8 - 4.5, ranged from 1 to 32mg/kg (Figure 1). At pH 4.5 the concentration of extractable Al ranged from 1 to 8mg/kg. This wide range of values is evidently not simply due to differences in pH and often indicates the influence of texture, exchange capacity and other properties of the soil. The effect of clay concentration on extractable Al was investigated. There was a positive relationship between clay and extractable Al values but this relationship was not consistent. We determined soil properties that may affect levels of extractable Al in soil including total organic carbon, cation exchange capacity and exchangeable cations but no single property is highly predictive of extractable Al and multivariate equations are only moderately predictive. As Al-toxicity reflects the concentrations of Al species in soil solution, the relationships between these concentrations, extractable Al and other soil properties have been investigated. The intent of this research is to develop a model or expert system for predicting the occurrence and potential development of Al toxicity based on those soil properties that are routinely determined in soil surveys.