We have investigated the room temperature adsorption of methylamine, dimethylamine and trimethylamine using density functional theory (DFT) and multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy. It was found that the reaction pathways of the amines resemble the precursor-mediated dissociative chemisorption of ammonia. Our calculations showed that although dissociation involving N-C bond cleavage is thermodynamically more favorable than the N-H dissociation pathway, the activation barrier for N-CH3 dissociation is significantly higher than that for N-H dissociation. This leads to selective cleavage of N-H bonds in the surface reactions of methylamine and dimethylamine, while trapping trimethylamine in its molecularly chemisorbed state through the formation of a Si-N dative bond. We also identified the products of the reactions of the amines on the Si(100)-2x1 surface by surface IR studies, confirming the theoretical predictions. The selectivity observed in the surface chemistry of simple model amines is briefly discussed in the context of organic chemistry at semiconductor surfaces.