The reactivity of enoxysilanes against aldehydes has been discussed theoretically in terms of the local electron-accepting ability and the Lewis acidic hardness of the silicon center. Those quantities have been evaluated by choosing a Si 3p atomic orbital that will play the major role in the formation of a new bond with the aldehyde oxygen and by projecting out the reactive unoccupied orbital for each enoxysilane molecule that shows the maximum amplitude on that AO. They change significantly depending on the arrangement of bonds around the silicon center. The activation energy calculated for the reaction of enoxysilanes with formaldehyde correlates well with both of the two quantities, showing that 1-enoxysilacyclobutane having a small C-Si-C angle is a stronger Lewis acid than methyl-substituted enoxysilanes. The acidic hardness of silicon in enoxysilane has been found to be very similar in magnitude to that of silicon in the corresponding allylsilane. In contrast, the gamma-carbon of an enoxy group has a higher electron-donating ability than that in an allyl group, suggesting that enoxysilanes are more reactive against aldehydes.