A method of preparing an aminoalkylalkoxysilane, the method comprising reacting a haloalkylalkoxysilane with ammonia in a high pressure reactor for an amount of time sufficient to consume from 20 to 99.99% (w/w) of the haloalkylalkoxysilane and form an aminoalkylalkoxysilane; venting ammonia from the reactor to give a mixture comprising the aminoalkylalkoxysilane, unreacted haloalkylalkoxysilane, and an ammonium halide; and treating the mixture with a primary amine to form an N-substituted aminoalkylalkoxysilane.