Catalyst activity and selectivity were tested in the redn. (100 degrees C/2 hrs.) of crotonaldehyde satd. vapors in 0.03 l. H-2/min in relation to the 5%Pd/SiO2 prepn. history, esp. to calcination-at 300, 500, or 700 degrees C in Ar and at 300 or 500 degrees C in H-2 just before the reaction. The 500 and 300 degrees C, resp., resulted in Pd distributed uniformly on the SiO2 surface, a factor crit. for catalyst's high activity. TOF-SIMS found CH3+, CH3O+, C2H5 and SiCH3+ on the catalyst surface, but calcination at 500 or 700 degrees C removed most of the org. fragments. The H,treatment at 500-700 degrees C, resulted in Pd clusters nonuniformly distributed (TOF-SIMS) on the surface and in a less active catalyst.