In order to elucidate the role of carbonaceous deposits, the hydrogenation of ethyne over silica supported Pt and Pd catalysts has been studied in a micro-pulse reactor. When a pure silica support (Aerosil) was used, the conversion decreased with an increasing number of ethyne pulses led over the reactor. Simultaneously, the selectivity towards ethene increased. When less pure silica (Kieselgel) was used as a support, the opposite effect was observed. These results are explained by assuming that the carbonaceous deposits are not active, but influence the selectivity of the uncovered metal surface by diminishing the average ensemble size available for the reaction. Impurities in the support can alter the behaviour of the catalysts.