Nitromethane, as probe molecule, and infrared spectroscopy, as analytical tool, were used to characterize the basic properties of alkali-exchanged X zeolites and metal oxides. It was observed that this probe molecule could adsorb molecularly, dissociatively and dissociative-associatively, according to the basic strength of the sites present on the studied solids. Alkali-exchanged X zeolites presented weak basic sites and a scale of basicity could be established, CsX > KX >NaX > LiX, based on the shift of the CH3 rock angular bending frequency. On Al2O3, ZrO2,TiO2 weak and intermediate basic sites were identified, while on MgO. La2O3 and alkali-promoted MgO just strong basic sites were found. For the alkali-promoted MgO samples no basicity scale could be observed. For the universe of the studied materials nitromethane behaves as a proper probe molecule to characterize basic sites with different strengths. (C) 2010 Elsevier B.V. All rights reserved.