The objective of this study is to provide a systematic assessment of the intraparticle mass transfer diffusional limitations in a Fischer-Tropsch synthesis (FTS) tubular fixed bed reactor under both conventional gas-phase media and supercritical phase media. Our results showed that the measured catalyst effectiveness factors are proportional to both the fugacity of CO in the bulk phase as well as to its effective diffusivity. The diffusional flux of CO in catalyst pores in the gas phase FTS was found to be higher than that of the supercritical phase FTS, but only at the entrance of the reactor bed. As the conversion increases, the reaction products completely fill the catalyst pores in the gas phase FTS. which results in drastic decrease in the CO concentration and diffusional flux in catalytic pores. Therefore, the catalyst effectiveness factor at the reactor exit drops more than 65% to that of the reactor entrance value. Nevertheless, in supercritical phase FTS, the variations in diffusional flux with conversion are not as intense, therefore the catalyst effectiveness factor along reactor length vary within 20% of the reactor entrance value. (C) 2012 Elsevier B.V. All rights reserved.