Investigation of the catalytic hydrolysis of isocyanic acid (HNCO) over TiO2 revealed that the rate of hydrolysis was considerably reduced in the presence of NO2 at temperatures below 200 degrees C. Transient low temperature investigations of the reaction, Temperature Programmed Desorption (TPD) experiments and Diffuse Reflectance Infrared Fourier Transform (DRIFT) investigations showed that part of the NO2 adsorbed strongly on the surface while part reacted with NH3 to form ammonium nitrate on the catalyst surface. Since at high temperatures NO2 was desorbed and NH4NO3 was decomposed, the catalytic activity was completely restored by thermal treatment at high temperature. (c) 2006 Elsevier B.V. All rights reserved.