New polymorphs of EuP5O14 and GdP5O14 ultraphosphates have been discovered and their crystal structures have been determined. The luminescence properties of the Eu3+ ion incorporated as a local structural probe for GdP5O14 have been investigated at room temperature. Display omitted •The crystal structures of new polymorphs of EuP5O14 and GdP5O14 have been determined from single crystals X-ray diffraction.•Structural properties of GdP5O14 polycrystalline thoroughly discussed by using XRD, FT-IR and Raman spectroscopy.•Photoluminescence excitation and emission spectra of GdP5O14:Eu3+ were measured. New polymorphs of EuP5O14 and GdP5O14 ultraphosphates have been discovered and their crystal structures have been determined from single-crystal X-ray diffraction. These two polymorphs crystallize with monoclinic symmetry in the space group C2/c (no. 15). They complement the series of type II ultraphosphates with the formula LnP5O14 known for heavier rare earths from Tb to Lu inclusively and including the homologous Y and Bi members. The structural properties of polycrystalline GdP5O14 have been thoroughly discussed with reference to X-ray diffraction results, Fourier-infrared and Raman spectroscopy. Under ultraviolet excitation, Gd1−xP5O14:xEu3+ (x = 5%, 10%) samples show a characteristic red emission originating from the 5D0 → 7F2 transition of the Eu3+ ion.Photoluminescence excitation and emission spectra of GdP5O14:Eu3+ were measured. The results show that it can be considered as a promising red phosphor for solid-state lighting and as such may have potential applications in atomic-like all-optical devices. The luminescence properties of the Eu3+ ion incorporated in the monoclinic C2/c polymorph as local structural probe show that in GdP5O14:Eu3+ powders, the Eu3+ ions are distributed over the two Gd3+ crystallographic sites of C2 symmetry of this structure.