The reaction of CO and H-2 on alumina-supported Pd catalysts (2, 5 and 10 wt% Pd) has been studied by a transient pulse technique in vacuum (Temporal-Analysis-of-Products (TAP) reactor). CHO species were formed on a hydrogen-treated catalyst surface at temperatures between 423 and 530 K by the interaction of hydrogen solved in the palladium metal and CO (adsorbed at 300 K after reduction). The CHO species were released into the gas phase when hydrogen was pulsed over the catalysts. CHO is assumed to be an intermediate in the hydrogenation of CO to CH4 starting at 560 K. No methanol was observed under the experimental conditions applied.