Volumetric adsorption of nitrogen and gravimetric adsorption of dioxan is reported for nonporous, mesoporous, and microporous silicas. Adsorption on the microporous material is governed by pore volume and dimensions. Adsorption on the nonporous silica and the mesoporous silica at up to monolayer coverages is governed by specific hydrogen bonding interactions between surface silanol groups and dioxan molecules. Different silanol group distributions for different silicas after standard treatment coupled with different possible modes of adsorption casts doubt on the use of nonporous silica as a standard for the study of interactions between porous silicas and H-bond acceptor molecules.