An innovative coating-by-parts (CBP) sampling method is proposed to improve the experimental study of water gas shift (WGS) systems used in systems in packed-bed and monolithic reactors. The CBP method enables detailed measurements of chemical species concentrations and operating conditions inside a reactor. Chemical kinetics-coupled CFD model has been set up for simulation to evaluate the performance of CBP. The model was first validated by the experimental data and subsequently run for multiple cases to simulate the use of CBP. The results show that the CBP method has an acceptable accuracy in packed-bed reactor and monolithic reactors. Meanwhile, another commonly used experimental method, namely, the central line sampling method, is found to achieve a high accuracy in the monolithic reactor, but a lower accuracy in the packed-bed reactor with low inlet velocity and packed by larger pellets. Copyright (C) 2010 John Wiley & Sons, Ltd.