The surface chemistry of silicon resonators plays an important role in mechanical energy dissipation. Significantly higher quality factors are observed for methyl-terminated-MHz-frequency silicon oscillators as compared to similar resonators terminated with longer alkyl moieties. Megahertz-frequency silicon micromechanical torsional resonators were coated with mixed monolayers consisting of methyl groups with a small admixture of functional moieties. Infrared absorption spectroscopy and X-ray photoelectron spectroscopy were used to characterize the functionalized surfaces. Resonators terminated with mixed acetal/methyl monolayers had very high quality factors. In contrast, when the acetal moiety was converted to an aldehyde using standard deprotection chemistries in both aqueous and organic media, a dramatic decrease in resonator quality was observed. Resonators terminated with a mixture of methyl and butenyl moieties were found to have high quality factors than those observed for methyl-terminated resonators. Furthermore, the terminal double bond on the butenyl moiety was reacted with the terminal double bond on ethyl 10-undecenoate molecule via olefin cross-metathesis, resulting in ester-terminated resonators. The resulting quality factor of the resonators was found to be acceptably high. Thus, a successful method for termination of resonators with reactive functional groups while maintaining high quality factor was established.