Three gas-phase oxidation reactors are studied : the reaction of sulfur dioxide to sulfur trioxide, o-xylene to phthalic anhydride, and n-butane to maleic anhydride. Mathematical models are developed to account for the transport phenomena in packed-bed reactors. A software package, the Chemical Reactor Design Tool, is used to solve these models and assess the effect of transport phenomena, Phenomena studied include heat and mass transfer resistances around the catalyst particles; density, velocity, and pressure variations; radial dispersion; and axial dispersion. Criteria are developed and taken from the literature to estimate the importance of these phenomena a priori. These criteria are compared with the results of the computer simulations. In most cases, the criteria accurately predict which phenomena are important to a particular reactor design. When the estimate from the criterion is questionable or borderline, computer simulations can be used to investigate the phenomenon’s significance.