The isomerization reaction of n-pentane over porous solid acid catalyst was investigated at 198.0 degreesC, which is slightly higher than the critical temperature of n-pentane. The pressure of the reactant in the reactor was controlled so that the reaction fluid was in gas, near critical, and supercritical (SC) states. The results showed that the conversion of n-pentane increased sharply with increasing pressure near the critical point. The amount of coke deposition depends strongly on the pressure of the reaction fluid. The main reason for the deactivation of the catalyst was coke deposition on the catalyst. Deactivation of catalyst was repressed at the higher pressure because more coke precursors produced in the reaction was dissolved into the fluid, which increases the stability of the catalyst. UV-vis, IR, thermogravimetric analysis and H-1 NMR studies indicated that there were at least two kinds of coke precursors on catalyst surface. The effect of pressure on the properties of the coke was not considerable, although it influences the quantity of the coke deposition on the catalyst significantly. The loss of sulfur can be neglected at reaction temperature and the state of sulfur remains unchanged after reaction. It is advantageous to conduct the isomerization reaction at SC condition. (C) 2002 Elsevier Science B.V. All rights reserved.