Semiempirical PM3 calculations systematically overstate the free energy of the second-order-reaction transition state of aromatic isocyanate reactions with monofunctional active hydrogen compounds. The corresponding reaction rate predicted by transition state theory is far lower than observed. It is found that PM3 results accurately predict quantitatively isocyanate reaction rates when calibrated calculated Gibbs free energies are applied to activated complex theory. Modeling is accomplished by fitting experimental data to the Arrhenius reaction rate constant expression obtained in PM3 calculations. Calibrating semiempirical transition state results with experimental data of phenyl isocyanate and carboxylic acids with varied ligands, it was possible to distinguish likely atomistic mechanisms from among several transition states that were fully characterized.