The chemical reactions that take place in non-thermal plasma plug-flow type reactors can change the number of particles, the volumetric flow rate and the concentration of species, even if they do not take part in chemical reactions themselves. These changes, however, are frequently neglected in the computation of parameters characterizing the chemical process thus introducing a systematic error in parameters such as conversion, selectivity, yield, efficiency, residence time, carbon balance and energy efficiency. We discuss errors arising from this neglect, the methods currently used to take these factors into account and propose a simple procedure to obtain correct values, based on an internal standard. This procedure is applied on methane conversion with CH4/CO2 or He/CH4/CO2 mixtures where we found that neglecting the volumetric flux change introduces errors of up to 20% on process parameters. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.