Three kinds of Cr(II) ions are present on CO reduced CrO 3 /SiO 2 samples (labelled A, B. and C), having two, three and four oxygen ligands before chemisorption, respectively. The population of Cr A , Cr B and Cr c on a standard sample is evaluated to be: 52%, 28%, 20%, respectively. The number of oxygen ligands determines the actual charge of the ions, increasing in the series A, B, and C, as well as the ability to coordinate further ligands. CrA (coinciding with that proposed by Krauss) yields, upon NO chemisorption, dinitrosylic species at an angle of 125°: the presence of a third coordinative vacancy at least allows coordination of CO leading to Cr A (NO) 2 CO and of pyridine, leading simultaneously to Cr A (NO) 2 Py and Cr A (NO)Py 2 . This latter species may be converted into Cr A (NO)Py. Cr B (for which a structure slightly different from Krauss’ one is proposed) forms dinitrosylic species at an angle of 123°, with spectral features very close to those of Cr(III)(NO) 2 : no reaction occurs with CO, and, by pyridine adsorption, a Cr B (NO)Py species is formed. Cr c ions form, at low NO pressures, mononitrosylic species undergoing ligand displacement with either CO or pyridine; at high NO pressures,Cr c (NO) 2 species are formed, at an angle of 109°, which show no reactivity with CO and a displacement reaction with pyridine: a species Cr c (NO)Py is however not formed. Cr A and Cr B behaviour is that of isolated species; Cr c ions show, by the constant presence of shifts in the relevant NO frequencies upon NO coverage, that they are in electronic interaction.