A kinetic study was conducted for hydrolysis and oxidation of HCN over limestone at 1123 K with a fixed bed reactor. In the absence of O2, HCN was hydrolyzed to mainly NH3. However, the reaction rate of hydrolysis was far lower than that of oxidation. In the presence of O2 at concentrations higher than 0.5%, oxidation of HCN was dominant and most of HCN was oxidized to NO, while only a small amount of NH3 was produced. Conversion of HCN to N2O for oxidation over limestone was far lower than the conversions which had been reported for homogeneous oxidation of HCN. © 1993, American Chemical Society. All rights reserved.