A portion of the hydrogen of a high-surface-area silica was exchanged with deuterium gas in a pulse flow microreactor. The exchange reaction was rapid at 190 โข C when the silica was physically mixed with a Pd on carbon catalyst. The amount of readily exchangeable hydrogen was determined using a titration technique to be 0.87 mmol H/g silica, more than 50% of the estimated number of hydroxyl groups. A smaller number, 0.46 mmol H/g silica, was determined after the silica was partially dehydrated by heating to 360 โข C. Although silica itself becomes an active catalyst for H 2 /D 2 exchange after being heated to 360 โข C, exchange between D 2 gas and the surface hydroxyls was not observed in the absence of the Pd/carbon catalyst. The results are interpreted in terms of hydrogen spillover from the Pd/carbon catalyst to the silica surface. Equilibrium is established for a pool of hydrogen including that in the gas phase, on the catalyst surface, and a certain portion of the hydrogen on the surface of the silica, providing evidence that transport by spillover is rapid in comparison to the transit time of a pulse, about 3 min. Similar results were found by substituting polyvinylphenol for silica. The results illustrate that ready pathways exist for transport of spillover hydrogen between solid substrates and physically admixed catalyst.