Through an analysis combining powder XRD, TGA, and F-19 and H-1 solid-state NMR, it is confirmed for NbO2F and shown for TaO2F that both contain hydroXyl, defects and, metal vacancies when prepared by aqueous solution. synthesis. The formulations M1-x square O-x(2)_(5x)(OH,F)(1+5x) of both The samples are determined. The effects of the usually applied thermal treatments are examined. Obtaining puke NbO2F and TaO2F from these samples, that is, fully removing metal vacancies and hydroxide, while avoiding the formation of M2O5, is not that easy. Since- thermal treatments result in dehydroxylation and defluorination, it requires, at least, a larger amount of fluorine than Metal may not be the case. We also confirm that the solid-state synthesis is an efficient method to avoid metal vacancies and hydroxyl defects in NbO2F and then apply it to the synthesis of TaO2F. The local structure of NbO2F and TaO2F is poody-clescribed by an ideal cubic ReO3-type model with 0 and F randomly distributed over the available anion sites. Since O/F ordering was previously highlighted, NbO2F and TaO2F cubic 3 X 3 X 3,Supercells featuring.-M-O-M-O-M-F- chains along. <100> have been built and geometry optimized. These optimized supercells lead to more realistic structures than the previously proposed models, that is, really disordered structures with angularly and radially distorted MX6 octahedra as expected in disordered,Compounds,Moreover the structural modeling of NbO2F and TaO2F by these geometrr-optiniized supercells is supported by the computed F-19 and Nb-93 NMR. parameters, which give very good agreement with the experimental ones.