Many transition-metal oxides in elevated valence states [e.g. Mn(V), Co(IV), Ni(III), Cu(III) ] present a metastable character and, given the difficulty of their synthesis, have been relatively little studied. However, they are very interesting materials presenting strong electronic correlations that are bound to exotic properties such as superconductivity, metal behavior, metal–insulator transitions or colossal magnetoresistance. The metastability of these compounds requires special synthesis conditions such as the application of high pressure. In the last years, we have prepared and investigated a good number of materials belonging to several families such as RNiO₃ (R = rare earths), Ba₃Mn₂O₈ , (Ba,Sr)CoO₃ , La₂(Ni,Co)O_4+δ , etc. In the study and correct characterization of these oxides it has been decisive the use of elastic neutron diffraction, most of the times in powder samples. This technique has allowed us to access the structural details typically related to the octahedral tilting in perovskite structures, the oxygen stoichiometry and order–disorder of the oxygen sublattice, the distinction between close elements in the Periodic Table, the resolution of magnetic structures and, in general, the establishment of a correlation between the structure and the properties of interest. This letter is organized around the binomial "high-pressure synthesis" and "characterization by neutron diffraction" and illustrated with some selected examples among the metastable materials above mentioned.