The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant C-13 kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C-2-C-3 and C-5-C-6 bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple SO heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N-2 by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the C-13 isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between SN2Ar and SO mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.