Imidinium ions, ArN CNR2+, were generated in aqueous solution from the solvolysis of fluoro and chloro formamidines at pH 9.3 and 25° C. Rate constants for the hydration of five imidinium ions were determined from a kinetic analysis of their trapping by thiolacetate anion, CH3COS-, in the presence of a pool of competing fluoride anion, and a rate constant of k AcS-= 5× 109 M-1 s-1 for diffusion-controlled trapping of the carbocations with ...