The adsorption of alpha-pinene to solid surfaces is an important primary step during the chemical conversion of this common terpene over mesoporous materials, as well as during the formation of atmospheric aerosols. We provide evidence of tight and loose physisorbed states of alpha-pinene bound on amorphous SiO2 as determined by their adsorption entropy, enthalpy, and binding free energies characterized by computational modeling and vibrational sum frequency generation (SFG) spectroscopy. We find that adsorption is partially (4060%) irreversible over days at 294342 K and 1 ATM total pressure of helium, which is supported by molecular dynamics (MD) simulations. The distribution of alpha-pinene orientation remains invariant with temperature and partial pressure of alpha-pinene. Using the RedlichPeterson adsorption model in conjunction with a vant Hoff analysis of adsorption isotherms recorded for up to 2.6 Torr alpha-pinene in 1 ATM total pressure of helium, we obtain Delta S degrees(ads), Delta H degrees(ads), and Delta G degrees(ads) values of -57 (+/- 7) J mol(-1) K-1, -39 (+/- 2) kJ mol(-1), and -22 (+/- 5) kJ mol(-1), respectively, associated with the reversibly bound population of alpha-pinene. These values are in good agreement with density functional theory (DFT)-corrected force field calculations based on configurational sampling from MD simulations. Our findings are expected to have direct implications on the conversion of terpenes by silica-based catalysts and for the synthesis of secondary organic aerosol (SOA) in atmospheric chambers and flow tubes.