We report on the formation of a molecule-ion adduct of β-cyclodextrin (β-CD) with sodium arsenite (SA) both in solid state and in aqueous solution. The adduct was characterized using Fourier transforminfrared spectroscopy, powder X-ray diffraction, differential thermogravimetric analysis, gas chromatography coupled to time-of-flight mass spectrometry, UV-Vis spectroscopy, 1H nuclear magnetic resonance titration and electrospray ionization mass spectroscopy. We found that a weak molecule-ion interaction was responsible for the difference in spectral properties in solid state and in aqueous solution as well as for the difference in thermal decomposition behavior between β-CD, SA and the adduct. The novelty and complexity of this interesting molecule-ion interaction was revealed by a redox reaction based on the data of gas chromatograph coupled to time-of-flight mass spectrometer and by the formation of the 1:1 (molar ratio) supramolecular ion complex, Na+-β-CD, based on its electrospray ionization mass spectrum.