We report on intense photoluminescence from materials of the (Rb,K)(2)Ge4O9:Mn4+ solid solution as a novel class of redemitting oxide phosphors. In these compounds, luminescence originates from a virtually ideal substitution of Mn4+ for Ge4+ on octahedral lattice sites which are well-isolated from each other within the unit cell by intermediate GeO4 species. Complete isostructural substitution of K for Rb is possible across the join. The associated slight shrinkage of the unit cell has only little effect on the apparent Mn4+ interionic distance, but enables tuning of the absorption cross section and of the band structure, hence, of the emission lifetime, of the excitation band shape, and of emission quantum yield. Partial substitution was also found to reduce thermal quenching of the Mn4+-related emission, apparently due to the lower polarizability of the K+ ion. In addition, random substitution of Rb by K enables modulation of the interaction of Mn4+ with its surrounding field at lower symmetry, leading to increasing emission bandwidth, i.e., 595 cm(-1) in K1.5Rb0.5Ge4O9:Mn4+ vs 558 or 578 cm(-1) in Rb2Ge4O9:Mn4+ or K2Ge4O9:Mn4+ respectively.