NH4[Co1-yNiyPO4]H2O (0y1) single crystals were fired to prepare Co2-xNixP2O7 0x2 solid solutions between 800 and 1200 degrees C. The structure of these solid solutions is -M2P2O7 (M = Co, Ni). The contraction of the unit cell with increasing x confirmed the formation of these solid solutions. The UV-vis-NIR spectra of these solid solutions were consistent with Co(II) and Ni(II) ions in the two different crystallographic sites (octahedral site and square pyramidal site) in the -M2P2O7 structure. Due to their thermal stability, Co2-xNixP2O7 (0x2) solid solutions with -M2P2O7 structure can be used as a blue, green, brown or yellow ceramic pigment.