Infrared spectroscopy is among the most widely used techniques for studies of industrial catalysts. Since many properties of catalysts depend intimately on the environment, it is important to carry out the studies under in situ conditions. The improved understanding obtained from such in situ IR characterizations has lead to developments of new and improved catalysts and catalytic processes. To illustrate the scope of applications, examples from three different industrial catalyst systems, i.e. DeNO(x), methanol synthesis and hydrotreating reactions, will be discussed. (c) 2005 Elsevier B.V. All rights reserved.