The silylation of functional groups plays an important role in making organic compounds more volatile for gas chromatography and mass spectroscopy and in protecting functional groups during further synthetic transformations. Using low temperature NMR, we have established that a complex is formed between a pyridine base and a silyl chloride. Equilibrium constants and thermodynamic parameters have been obtained for complex formation between various silyl chlorides and 4-substituted pyridines. This complex is probably the active silylating reagent and could explain why one functional group is selectively silylated in the presence of other similar functional groups.