Operando spectroscopy is a methodology that combines the spectroscopic characterization of a catalytic material during reaction with the simultaneous measurement of catalytic activity/selectivity. Operando spectroscopy is a new term that has been introduced into the catalysis literature at the beginning of the 21st century that underlines the importance of simultaneously combining fundamental spectroscopic surface phenomenon with catalytic performance, on the same sample, under realistic reaction conditions. The potential impact of operando spectroscopy on catalysis science lies in its ability to significantly assist in the establishment of fundamental molecular structure-activity/ selectivity relationships for catalytic systems. It is critical that the design of the spectroscopic reaction cell, the catalytic reactor, also allows for the generation of catalytic performance data that is analogous to those achieved with conventional catalytic reactors. A case study of operando Raman-GC study is presented along with consideration of the methodology, spectroscopy and samples. (c) 2004 Elsevier B.V. All rights reserved.