Interfacing multifunctional molecules with semiconductor surfaces has a variety of applications; however, it is important to understand the selectivity of target surface reactions to produce the surface with desired functionality. In this work, a silicon surface modified with 1-amino-10-undecene was reacted with [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) in toluene. Two possible competing reactions for PCBM, via the ester group and by direct attachment to the C-60 portion, are analyzed. X-ray photoelectron spectroscopy, atomic force microscopy, Fourier-transform infrared spectroscopy, and time-of-flight secondary ion mass spectroscopy, supplemented by density functional theory calculations, suggest that despite the similarity of the energetics for those two reaction pathways, predominant chemisorption occurs via the direct attachment of the C-60 cage to the primary amino group of the functionalized silicon surface.