Microcalorimetric measurements, infrared spectroscopic studies (FTIR), and quantum-chemical calculations based on density-functional theory (DFT) were made of the interactions of isobutene with silica at 300 8. On the basis of DFT calculations and FTIR spectra, most of isobutene adsorbs reversibly on silica at 300 K, involving the interaction of the pi-bond with hydroxyl groups on the surface. The average energy of these interactions is similar to 45 kJ/mol at a surface coverage of similar to 400 mu mol of isobutene per gram of silica. The formation of butoxy and 2-methyl-1-propoxy species upon reaction of isobutene with silanol groups appears to be limited kinetically at 300 K. While the enthalpies of formation of these species from gaseous isobutene and silanol groups are calculated to be -55 and -40 kJ/mol, respectively, the activation energies for the formation of these species from adsorbed isobutene are estimated from DFT to be 172 and 226 KJ/mol, respectively. These high activation barriers are caused by the required localization of positive charge in the corresponding transition states, which is made difficult by the weak acidity of silica. Minor amounts of surface butoxy might form on silica at 300 K, perhaps on defect or impurity sites, and these species may be responsible for the higher heats of adsorption (56 +/- 2 kJ/mol) measured at low surface coverages on silica.