An analysis process is provided for the determination of a complete material balance of a chemical reaction, especially useful in the studies of catalyst activity. A complete material balance is obtained by determining the output flow from a reactor at the temperature and pressure conditions of the chemical reaction. The reactor output flow is determined by first analyzing the effluent volume composition at the temperature and pressure conditions of the reaction. Subsequently, the effluent is expanded and the volume composition and flow rate of the expanded effluent are measured. From these measurements and the volume composition at the reactor output, the output flow from the reactor is determined. A strict material balance is then possible by comparing the input flow rate and volume composition to the output volume composition and flow rate, thereby establishing a complete material balance at the reaction temperature and pressure. The process is suitable for testing the catalytic activity of even small amounts of catalysts independent on the catalyst shape. Moreover, the process provides a high degree of reproducibility through the automation of temperature control and the analysis of volume compositions and flow rates.