Atomic doping is one strategy known to induce half-metallicity, improve thermal stability, increase conductivity and increase magnetoresistance. Solid solutions -where dopant composition can be continuously varied between two parent compounds -provide even greater flexibility in tuning physical properties. One flexible approach towards solid solutions are mixed-anion materials, containing multiple anions in a single phase. This approach offers a route towards tuning composition while maintaining the cationic interactions important for many properties, however, the number of such materials is currently limited. Niobium oxyfluorides are one particular class of mixed-anionic materials of interest for functional applications: the variable oxidation state of niobium; similar ionic radii of oxygen and fluorine; and higher covalency of Nb-O bonds cf. Nb-F bonds leads to direct control of physical properties through variable oxygen:fluorine ratios. These materials are built around corner-sharing Nb(O,F)6 octahedra, with a number of shear-related phases also known. The simplest structure observed in the family is that of cubic ReO3 (Fig. ) and this structure has been reported across the entire NbO2-xF1+x solid solution, ranging from x = 0 (Nb V O2F) to x = 2 (Nb III F3). Such ReO3-type (oxy)fluorides are of wide interest, with a range of properties including lithium incorporation for battery applications or negative thermal expansion as in ScF3. The latter property is linked to a cubic to rhombohedral transition apparent in many ReO3-type trifluorides, caused by octahedral rotation around the [111] crystallographic direction (a -a -a -in Glazer's tilt notation). Such a transition has also been observed in NbO2F under a relatively low hydrostatic pressure of 0.47 GPa (Fig. ). 24 Electronically, NbF3 has been predicted to be half-metallic, with a large minority spin band gap (8.9 eV) that would minimise spin flipping within a real spintronic device. In contrast, NbO2F is known to be a wide band gap semiconductor with a band gap of 3.4 eV. However, there is no information on the electronic structure of the intermediate solid solutions. It is therefore unclear how half-metallicity arises in NbF3 and also how the band structure is modified to reach semiconducting NbO2F. Here, we report electronic structure calculations across the NbO2-xF1+x solid solution to understand the relationship between electronic structure, half-metallicity and anionic composition. In addition, we describe the effect of anionic composition on the cubic to rhombohedral phase transition under applied pressure.