With use of density functional methods and large basis sets, as implemented in the Jaguar v. 3.0 ab initio electronic structure package, we calculated the activation energy of the 1,3-dipolar addition of phenylazide to 19 different reactants containing a carbon-carbon double bond. The results provide an excellent prediction of the relative reaction rates observed experimentally as the substituents of the reactant are varied. Regioselectivity of the products of the reaction also are predicted reliably by the calculations. The use of a large experimental data set and full representation of the reactant species (as opposed to smaller model systems) provided a high degree of confidence in the ability of the electronic structure methods to reproduce experimental data reliably. The effects of method of geometry optimization and inclusion of solvation effects were investigated and found to be relatively small.