The phenomenon of capillary condensation is studied through the weight change of condensed phase in an internally wetted porous pd catalyst pellet during the course of exothermic reacation of cyclohexane hydrogenation in a thermogravimetric analyzer (TGA). The direct measurement of condensed phase indicates that evaporation of condensed phase is confined to the macropores of catalyst pellet at the lower limit of reaction rate. Phase transition from lower to upper limit reaction rate associates with the sharp temperature rise in the catalyst pellet . A substant ial fract ion of pellet pores volume opens in the phase t ransit ion and evaporation of condensed phase extends to the micropores of catalyst pellet. The TGA studies shows the reacation rate multiplicities is the result of condensation - evaporation hsteresis associated with the internally wetted catalyst pellet.