As a result of this research, twenty-four selenites of different transition metals were synthesised. These were grouped as a function of the conditions of the pressure and the temperature employed in their synthesis, differentiating three blocks of eight phases each. New polymorphs obtained were: Mn(SeO ).H O(A,B), in low pressure and temperature conditions, Mn (SeO )3.H O and Mn (SeO )4.H O phases in moderate pressure and temperature conditions, and Mn(SeO )(A,B) polymorphs in high pressure and temperature conditions. The aims were, apart from obtaining new selenites, the undertaking of a structural analysis by means of X-ray diffraction in monocrystalline and in polycrystalline samples. The studies carried out enabled the elucidation of the crystallographical parameters for each phase. The X-ray diffraction diagrams for the polycrystalline sample showed a significant absence of impurities. Finally, the structures of twenty-four phases, obtaining the link distances, the distortion and the equilibrium of the loads of the units present, were investigated. Studies were undertaken of thermal properties (thermogravimetric and thermodiffractometric), spectroscopic properties (infrared, diffuse reflectance, luminescence and Mössbauer) and magnetic properties (magnetic susceptibility and electronic paramagnetic resonance) of the selenites obtained. The thermogravimetric and thermodiffractometric analyses show an initial loss of the water molecules and the posterior decomposition of all phases. After elimination of selenium in the form of SeO (g), the formation of simple oxides of the metals present in the initial structures takes place. The analyses of the infrared spectra of the selenites show the characteristic bands for the (SeO ) selenite groups, in the interval of frequencies from 1000 to 400 cm . The diffuse reflectance spectra present the characteristic bands of cations, in all case obtaining a certain degree of covalency. Manganese compounds show luminescence phenomena at temperatures of liquid He. The emission and excitation spectra present bands belonging to manganese(II) ions in high-spin octahedric environments. Magnetic measurements indicate the presence of predominantly antiferromagnetic interactions, with a part of the samples showing ferromagnetic phenomena at low temperatures. One of the synthesised phases, selenite with the Cu O(SeO ) formula, present global ferromagnetic behaviour.