We have developed a two-step method using preadsorbed ethylenediamine (EDA) to catalyze the reaction of (3-aminopropyl)dimethylethoxysilane (APDMES) on silica surfaces. The amount of adsorbed APDMES is nearly twice the amount obtained when the same reaction is performed without the preadsorbed EDA or when monoamines such as propylamine or triethylamine are used instead of EDA. In addition, the use of preadsorbed EDA leads to an increase in the number of chemisorbed APDMES with free aminopropyl groups extending from the surface. Preadsorbed monoamines such as triethylamine, pyridine, or propylamine do not alter the adsorbed amount or change the number of free amine groups extending from the surface because they are easily displaced from the surface by APDMES. Infrared spectroscopy is used to follow the surface reactions on silica powder, and contact angle measurements show that this method also works on glass slides.