<jats:p>The 51V NMR of the diamagnetic V5+ and paramagnetic V4+ ions has been studied in the semiconducting V2O5–P2O5 glass system. From computer-simulation studies of the 51V NMR experimental spectra due to the V5+ sites, it has been determined that the spectra obtained at the higher spectrometer frequencies can be described by a skewed distribution of powder patterns arising from axially symmetric chemical shifts. The most probable value of the chemical shift distribution (Σ‖)mp of the chemical shift parameter Σ‖ decreases with increasing V2O5 content and (Σ⊥)mp is independent of glass composition. An estimate of the nuclear quadrupole coupling constant of the V5+ site has been made; it ranges from 1.0 to 1.5 MHz as the V2O5 content of the glass is increased. The NMR data are interpreted in terms of two types of VO5 units in the glass. One is a VO5 unit similar to that in crystalline V2O5 and the other is a VO5 unit with a PO4 unit substituted for the apex oxygen. From ESR intensity measurements, the ratio of the fractions of V4+ and V5+ ions (n4 / n5) present in the glass has been determined as a function of glass composition. There is some evidence to indicate that two V4+ sites are formed for each PO4 unit that is substituted for an apex oxygen of a VO5 unit. The implications of these data for the electrical conductivity of the glasses are discussed.</jats:p>