Rate constants for the hydrolysis of 2-(2'-imidazolium)phenyl hydrogen phosphate (IMPP) in water at pH<6 indicate that activation by the imidazolium moiety disappears with the deprotonation of the phosphate group, and the reaction involves the hydrogen-bonding of the imidazolium NH with the aryl oxygen leaving group. The reaction should involve a near-planar conformation of the imidazolium and the phenyl groups in the activated complex, which favors proton-transfer. The crystal structure of IMPP was solved, and a bond length-reactivity correlation for reactions of phosphate monoester monoanions is described.