Imidinium ions, ArN = CNR2+, were generated in aqueous solution from the solvolysis of fluoro and chloro formamidines at pH 9.3 and 25 degrees 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 x 10(9) M-1 s(-1) for diffusion-controlled trapping of the carbocations with thiolacetate anion. The rate constants, k(5), for hydration of the imidinium ions, XArN=CNC4H8O+ are 3.6 x 10(5), 5.8 x 10(5), 1.5 x 10(6), 1.6 x 10(6), and 1.8 x 10(6) s(-1) for X = H, 4-Cl, 3-CN, 4-CN, and 3-NO2, respectively. In a similar experiment a rate constant of k(s) = 3.3 x 10(7) s(-1) was obtained for hydration of the imidinium ion 4-NO2-ArN=CNCH3(OCH3)(+), by using azide anion to trap the cation and a rate constant of k(az) = 5 x 10(9) M-1 s(-1) for diffusion-controlled trapping of the imidinium ion with azide ion. The partitioning rate constant ratio k(AcS)-/k(s) for 4-ClArN=CNC4H8O+ decreases by approximately 6-fold in 45% v/v glycerol/water in contrast to k(az)/k(s), which remains almost constant, showing that thiolacetate, but not azide anion, combines with the imidinium ion at a diffusion-limited rate. The reactivity of the imidinium ions deviate from behavior predicted by the N+ scale in a manner that may be explained by a difference in the selectivity of the cation when compared to the more stable N+ carbocations.