This paper is concerned with the study of decay of industrial catalysts, both in the plant and in the laboratory. Techniques are described for deciphering the characteristics of catalyst decay in the industrial plant, using plant performance data and post-mortem characterization. Simulated examples are used to show how such data can be used to construct simple decay models. The detailed investigation of decay kinetics in the laboratory usually requires the use of gradientless reactors. However, even studies in a single-tube, plug-flow reactor can provide useful insight into decay kinetics, especially if segmental catalyst discharge and recharge is possible. With parallel testing in multi-tubular catalyst test units there are even greater possibilities. Accelerated decay techniques are frequently used in industry for comparative evaluations of catalyst performance. Accelerated decay is a valuable tool but only if it is used wisely. A simple index is proposed for the purpose of judging the validity of test procedures for accelerated decay. (C) 2003 Elsevier B.V. All rights reserved.