In most industrial applications of isocyanates, the parent HNCO species is often substituted with some much larger RNCO species. The most common -R groups are generally large aromatic rings or long polymers which can both be modified to possess a diverse array of electronic structure features. Therefore, it is necessary to understand how different substituents influence the electronic structure and the resulting energetics of the RNCO + H 2 O reactions. The ∆H 0K barrier heights and relative product enthalpies are predicted for a diverse set of 24 different substituted RNCO molecules proceeding through TS1 and TS2.