Tetragonal Er 0.5 Nb 0.5 O 2 and monoclinic ErNbO 4 micro-and nanoparticles were prepared by the citrate sol-gel method and heat-treated at temperatures between 700 and 1600 • C. ErNbO 4 revealed a spherical-shaped crystallite, whose size increased with heat treatment temperatures. To assess their optical properties at room temperature (RT), a thorough spectroscopic study was conducted. RT photoluminescence (PL) spectroscopy revealed that Er 3+ optical activation was achieved in all samples. The photoluminescence spectra show the green/yellow 2 H 11/2 , 4 S 3/2 → 4 I 15/2 and red 4 F 9/2 → 4 I 15/2 intraionic transitions as the main visible recombination, with the number of the crystal field splitting Er 3+ multiplets reflecting the ion site symmetry in the crystalline phases. PL excitation allows the identification of Er 3+ high-energy excited multiplets as the preferential population paths of the emitting levels. Independently of the crystalline structure, the intensity ratio between the green/yellow and red intraionic transitions was found to be strongly sensitive to the excitation energy. After pumping the samples with a resonant excitation into the 4 G 11/2 excited multiplet, a green/yellow transition stronger than the red one was observed, whereas the reverse occurred for higher excitation photon energies. Thus, a controllable selective excited tunable green to red color was achieved, which endows new opportunities for photonic and optoelectronic applications.