If the temperature level or free stream reactant concentration is deliberately changed, the resulting change in the observed global reaction rate on a catalytic surface in a flow system will not necessarily reflect the true chemical kinetics at the fluid/solid interface. General expressions, reminiscent of those applicable to a static system, are derived for the relation between the diffusional falsification of activation energy and reaction order in terms of a logarithmic derivative of the isothermal diffusion correction (effectiveness factor). Approximate solutions are given for the case of the thin, nonturbulent diffusion layer which develops along an impermeable catalytic flat plate, for arbitrary values of the true reaction order and Prandtl number for diffusion (Schmidt number). Comparisons with exact solutions to the boundary-layer equations and alternate approximate methods are given for the special case of first-order surface reactions. Of the various quantities of interest in the diffusional theory of heterogeneous reactions in flow systems, it is shown that the accuracy of the Frank-Kamenetskii quasi-stationary method can become unacceptably poor for the calculation of these falsification parameters. The physicochemical conditions under which these errors are likely to be largest are discussed. Applications are given to the study of the chemical kinetics of fast surface reactions.