Gas-phase N2 adsorption of mass-selected ions of carbohydrates containing 2–5 hydroxy groups was investigated using a tandem mass spectrometer equipped with an electrospray ionization source and cold ion trap. The maximum quantities of adsorbed N2 molecules observed in the mass spectra were consistent with the quantities of hydroxy groups in the carbohydrates. The N2 molecules were adsorbed on the hydrogen atoms of the hydroxy groups of the carbohydrates. The hydroxy groups of the carbohydrates were quantified using gas-phase N2 adsorption. Gas-phase N2-adsorption mass spectrometry detects free hydroxy groups on the surface of gas-phase ions.