A high amorphous aluminum or iron oxide content in drinking water treatment residuals (WTRs) can result in a high phosphorus (P) sorption capacity. Therefore, WTR may be used beneficially to adsorb P and reduce P loss to surface or ground water. The strong relationship between acid ammonium oxalate-extractable aluminum (Al(ox)) and Langmuir phosphorus adsorption maximum (P(max)) in WTR could provide a useful tool for determining P(max) without the onus of the multipoint batch equilibrations necessary for the Langmuir model. The objectives of this study were to evaluate and/or modify an acid ammonium oxalate extraction of Al(ox) and the experimental conditions used to generate P adsorption isotherms to strengthen the relationship between Al(ox) and P(max). The oxalate extraction solution to WTR ratio varied from 40:1, 100:1, and 200:1. Batch equilibration conditions were also varied. The WTR particle size was reduced from <2 mm to <150 microm, and batch equilibration was extended from 17 h to 6 d. Increasing the solution to WTR ratio to 100:1 extracted significantly greater Al(ox) at levels of >50 mg Al kg(-1). No additional increase was found at 200:1. Reducing WTR particle size from <2 mm to <150 microm increased P(max) 2.46-fold. Extending the equilibration time from 17 h to 6 d increased P(max) by a mean of 5.83-fold. The resulting empirical regression equation between the optimized Al(ox) and P(max) (r(2) = 0.91, significant at the 0.001 probability level) may provide a tool to estimate the P(max) of Al-based WTR simply by measuring Al(ox). The accurate determination of WTR P(max) and Al(ox) is essential in using WTR effectively to reduce P loss in runoff or to reduce the solubility of P in agricultural soils or organic waste materials (biosolids, manure).