Catalyst aging by coke formation has been studied for the selective hydrogenation of acetylene in the presence of excess ethylene on supported palladium catalysts. Deposited coke was found to have a substantial influence on the effective diffusivity, which decreased about one order of magnitude during 100 h of operation. As has been observed previously the selectivity for the undesired ethane was higher on aged catalysts, while the activity for acetylene hydrogenation was almost constant. These effects, however, were strongly dependent on the catalyst particle size, although the behaviour of fresh catalysts was unaffected by mass transfer limitations. When the catalyst used was Pd/alpha-Al2O3 the change in selectivity with aging could be explained solely as a consequence of the increased diffusion resistance, The mass transfer effects were important also on Pd/gamma-Al2O3, but on this catalyst there was an additional increase in ethane selectivity that could not be attributed to diffusion limitations. Calculations and experimental tests showed that the observed phenomena are relevant also for the shell-type catalysts normally used industrially. The coke formation itself was about four to five times faster on Pd/gamma-Al2O3 compared to the alpha-Al2O3-supported catalyst. The coke was generally concentrated towards the pellet periphery showing the influence of diffusion resistance also on the coke-fonning reactions.