The structures of xFeO · (1−x)P2O5 glasses (0x0.50) prepared in sealed silica ampoules have been investigated using Mössbauer and infrared (IR) spectroscopies. Mössbauer results indicate that the glasses contain 6-18% Fe(III). Both Fe(II) and Fe(III) ions occupy octahedral coordination environments in these glasses. Analysis of the IR spectra indicates that the structure of glass transforms systematically from a three-dimensional P2O5 network (dominated by Q3 tetrahedra) to a two-dimensional network (dominated by Q2 units) as the FeO content increases. This transformation is evidenced by the replacement of the IR band at 1380 cm−1 attributed to P=O vibrations on isolated Q3 sites with the band at 1260 cm−1 band which is attributed to asymmetric vibrations of PO2 groups on the Q2 tetrahedra. In addition, a new IR band at 1317 cm−1 is created when FeO is added to P2O5 and this band is assigned to P=O bonds that participate in the coordination environments of the Fe-octahedra. At x0.35 FeO, the P=O bonds that remain are only those that coordinate with the Fe-octahedra. These structural changes are similar to those observed for other ultraphosphate glasses.