The purpose of this article is to analyze critically and quantitatively the effect of fluid reactant supply rates and mass transfer on the measurement of the rates of fluid-solid reactions, particularly those with small equilibrium constants. It is shown through a mathematical analysis that the measurement of the intrinsic kinetics of a reaction with a small equilibrium constant (a positive standard free energy of reaction) requires much larger rates of fluid reactant supply and mass transfer rates than that of a reaction with a large equilibrium constant. The overall reaction rate of the former also tends to be slow. Furthermore, the apparent activation energy of such a reaction approaches the standard enthalpy of reaction (DeltaH(0)), rather than the true activation energy of the chemical reaction. (C) 2004 Elsevier Ltd. All rights reserved.