Keggin-type aluminum oxyhydroxide species such as the Al-30 (Al30O8(OH)(56)(H2O)(26)(18+)) polycation can readily sequester inorganic and organic forms of P(V) and As(V), but there is a limited chemical understanding of the adsorption process. Herein, we present experimental and theoretical structural and chemical characterization of [(TBP)(2)Al-2(mu(4)-O-8)(Al-28(mu(2)-OH)(56)(H2O)(22))](14+) (Thp = t-butylphosphonate), denoted as (TBP)(2)Al-30-S. We go on to consider the structure as a model for studying the reactivity of oxyanions to aluminum hydroxide surfaces. Density functional theory (DFT) calculations comparing the experimental structure to model configurations with P(V) adsorption at varying sites support preferential binding of phosphate in the Al-30 beltway region. Furthermore, DFT calculations of R-substituted phosphates and their arsenate analogues consistently predict the beltway region of Al-30 to be most reactive. The experimental structure and calculations suggest a shape reactivity relationship in Al-30, which counters predictions based on oxygen functional group identity.