This work highlights for the first time the photoluminescence (PL) properties of two new [Ln(Mo8O26)2]5- (Ln = Eu, Sm) lanthanide-containing polyoxometalates. Stable crystals of their tetrabutylammonium salts were synthesized, and their structures were confirmed by single-crystal X-ray diffraction. The robustness of the [Ln(Mo8O26)2]5- complexes in an acetonitrile solution has been evidenced by Fourier transform Raman and PL spectroscopies. The tetraphenylphosphonium derivatives were obtained by a salt metathesis reaction. The two series exhibit high thermal stability in air and are efficient phosphors at room temperature.