The concept of effectiveness factor (r)) as a measure of diifusional limitations for gas-solid catalytic reactions has gone a long way since the time of Thiele. Multiple steady states giving rise to multiple values of (q) for the same bulk conditions, and r) values greater than unity have been widely reported in the literature in the last three decades. In this paper an interesting phenomenon associated with the effectiveness factors (q) for industrial gas-solid catalytic reactions is reported, that is the possible occurrence of negative values of q for certain intermediate components. This physically means that diifusional resistance can also reverse the direction of the net production or consumption of intermediate components in consecutive and/or reversible reaction networks. It is shown both numerically and analytically that the results represent real physical phenomenon and not artefacts resulting from numerical problems or model simplifications. Two industrially important reactions are considered, namely, the st-reforming of natural gas which is a highly endothermic reaction and the partial oxidation of 0-xylene to phthalic anhydride which is a highly exothermic reaction. NOMENCLATURE aa Ci Ci,B Ci,s Dij Dik Df Specific external surface area of catalyst pellet, m2/m3 Concentration of component, i kgmol/m3 Concentration of component i at bulk condition, kgmol/m3 Concentration of component i at pellet surface, kgmol/m3 Binary diffusivity of component i in component j, m*/hr Knudsen diffusivity of component i, m* /hr Effective diffusivity of component i, (-AHj) hi -kl ,ks E2 Ki m* /hr Df,mix Diffusivity of component i in a gas mixture, m*/hr h External heat transfer coefficient for catalyst pellet, kJ/m* .hr.OK t Author to whom all correspondence should be addressed. Heat of the jth reaction, kJ/kgmol Flate constant for the ilh reaction in the 0-xylene partial oxidation network kgmol/m3 of catalyst bed.hr kPa* Rate constant for reactions 1,3 (steam reforming reactions) kgmol.kPa".5/kg catalyst&r Rate constant for reactions 2 (steam reforming reactions), kgmol.kPa-' /kg catalysts.hr Adsorption equilibrium constant for component i, kPa_' Adsorption equilibrium constant for Hz0 Equilibrium constant for reaction 1 (steam reforming) kPa*