We studied the adsorption of (17-aminoheptadecyl)trimethoxysilane (AHTMS; NH2(CH2)17Si(OCH3)3) onto hydroxylated Si(100) wafers, which were preexposed to methyl-terminated n-octadecyltrichlorosilane (OTS; CH3(CH2)17SiCl3) for various immersion periods, resulting in partial coverages between 42% and 88% of a complete monolayer. Preadsorption of a partial monolayer of OTS was found to drastically improve the molecular alignment of subsequently adsorbed AHTMS. This effect was observed for submonolayer OTS films exhibiting both an island or a homogeneous growth mode, as determined by atomic force microscopy. Without preadsorbed OTS, the adsorption of AHTMS from an aged solution typically resulted in films of multilayer thickness, while the use of fresh solutions of AHTMS resulted in disordered films of submonolayer coverage. We suggest that submonolayer quantities of preadsorbed OTS prevent coagulated aminosilane adsorption and enable oriented adsorption of aminosilane molecules along the domain perimeter of aligned and ordered methyl-terminated silane. In comparison to sequential adsorption, coadsorption of n-octadecyltrimethoxysilane (OTMS; CH 3(CH2)17Si(OCH3)3) with AHTMS from a mixture gave films of low quality. The OTS/AHTMS surface that was prepared by sequential adsorption was exposed to pentafluorobenzaldehyde (PFBA) to probe the reactivity and orientation of the AHTMS amino groups. Eighteen to thirty-four percent of the amine groups reacted with no or minor disturbance to the molecular order, compared to 21-44% on pure and disordered AHTMS multilayers. This indicates that the coupling reaction with PFBA in toluene involves only part of the amino groups at the interface.