The effect of high pressure on the liquid-solid mass transfer coefficient (k(ls)) in a trickle bed reactor was investigated for the first time using the electrochemical method. It was found that k(ls) increases noticeably at high pressure and high gas flow rate. As superficial liquid mass velocity increases, k(ls) increases for all pressures and superficial gas velocities studied. The effects of reactor pressure and gas flow rate on k(ls) follow the trend proposed by Al-Dahhan and Dudukovic(1,2) based on five limited cases. The current correlations, which were developed based on atmospheric data, failed to predict the k(ls) values measured in this work as well as the trend of the reactor pressure effects.