The chemical interaction of adsorbed trimethylgallium and atomic hydrogen was investigated on a powdered silicon dioxide substrate containing various coverages of hydroxyl groups. The adsorption of trimethylgallium occurs nondissociatively at 128 K. Desorption of trimethylgallium occurs near 160 K by a nondissociative route. Atomic hydrogen, produced on a hot tungsten filament, reacts with adsorbed trimethylgallium at 128 K. This reaction produces infrared absorbances characteristic of dimethylgalliumhydride dimers or higher polymers, with Ga-H-Ga bridge bonds with a broad absorbance centered at 1690 cm(-1). These hydrogenated species are much more resistant to subsequent reaction with atomic hydrogen than trimethylgallium. Variations of the surface hydroxyl coverage on the silicon dioxide had no effect on the reaction of trimethylgallium with atomic hydrogen. Surface hydroxyl groups do not react with trimethylgallium below 173 K. The highly dehydroxylated silicon dioxide surface also does not react with trimethylgallium below 173 K.