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The catalyzed hydrolysis of HCN has become one of the most promising methods for the purification of HCN emissions. Three types of reaction mechanisms (named path A, B and C) for HCN hydrolysis were considered, and a total of 51 geometries including 20 transition states were optimized using ab initio MP2 methods with the 6-31++G(d,p) basis set. They share the first step of water attack for path A and B. In the following process, the proton of oxygen atom shifts to the nitrogen atom first for path A, while in path B, the proton of carbon atom shifts first. The path C contains the structural tautomer interconversion from HCN to HNC, and it turns out to be the most favorable pathway. Additionally, the water-assisted hydrolysis reaction mechanisms were examined for the three types of reaction processes. The inclusion of the auxiliary water decreases most of the Gibbs free energy barriers, and the formamide is the most stable intermediate on the free energy surface. The Gibbs free energy barrier of w-path A (50.39 kcal/mol) becomes the lowest. It is found that the process with the transfer of H atom from the C atom to the N atom is the rate-controlling step, and the efficient catalyst should activate C–N bond and assist the proton transfer. This information may help in designing new catalysts for this important reaction. [ABSTRACT FROM AUTHOR]