Cab-O-Sil fumed silica was modified in nonaqueous solutions with a variety of alkyltrialkoxysilanes under various reaction conditions in the presence and absence of an amine catalyst. Factors taken into consideration include physisorbed water and silane alkyl chain length. Unassociated silanol loss is discussed as an indication of silane reaction. A decrease in the intensity of the unassociated silanol infrared absorption band should not be used as a measure of silane reaction to the surface. Surface adsorbed water influences the amount of silane coverage both in the presence and absence of an amine catalyst. The advantage of an amine catalyst is lost with increasing surface-adsorbed water resulting in decreased surface silane loading. Cab-O-Sil equilibrated at 50% relative humidity and reacted in the presence of an amine catalyst produced the most silane loading for short chain alkyltrialkoxysilanes, whereas less silica surface moisture is optimal for reaction with longer chain alkyltrialkoxysilanes.