The steady-state flow method has been widely used in the kinetic study of heterogeneous catalysis. Under the conditions of steady state, all elementary steps in series are progressing at the same rate and hence the measured rates hardly tell the precise kinetic structure, or mechanism, of catalytic reactions which usually consist of several elementary steps including adsorption of reactants, surface reaction, and desorption of products. In the kinetic analyses of any given reactions, it is a common practice for the investigator to postulate a number of suspected rate models and then to assess these models to verify a optimal one which best fits observed rate data. Usually, however, there are quite a large number of plausible rate models even for a rather simple reaction, and it is not easy to determine the most adequate model among others even with the help of mathematical model indentification techniques with a electronic computer. This method is based on the assumption that there is a best model which reflects the true mechanism of the reaction sequences among the postulated models. But there is no certification which justifies this assumption. Therefore, even when a rate model is found to fit the rate data very well, the value of parameters appearing in the rate model should be further assessed in regards to their physical significance.