Transmission IR spectroscopic study at 298 K has revealed strong H-bonds between Lewis bases (LBs) containing one, [1-O], or two, [2-O], sp 3 hybridized O atoms and the surface OH groups of Cab-O-Sil. LB [ 1 - O ] (10 torr = 1.333 kPa) caused a greater Δν(OH) (470-520 cm - 1 ) than did LB [ 2 - O ] (385-470 cm - 1 ). In contrast with expectations, the intensity of the OH band at 3660 cm - 1 decreased to a greater extent for LB [ 2 - O ] than for LB [ 1 - O ] . The results were interpreted on the basis of charge-transfer theory. The estimated sequence of electron-donating ability (EDA) was: oxepane > oxane > oxolane > diethyl ether =< 1,4-dioxane > 1,3-dioxepane > 1,3-dioxane > 1,3-dioxolane.