The mixed niobate-borate crystals KNbB 2 O 6 and RbNbB 2 O 6 are monoclinic with space group Pna2 1 and point group m. Some of their properties are favourable for applications in non-linear optics including exceptional resistance to laser damage, UV cut-off at wavelengths below 290 nm and large d-coefficients. Although they have refractive indices that are similar to KTiPO 5 their birefringences are smaller than anticipated for structures containing planar borate rings and distorted NbO 6 octahedra, leading to large acceptance angles for second harmonic generation (SHG). Type 1 phase matching has been observed at 532 nm in RbNbB 2 O 6 . The birefringence of KNbB 2 O 6 is smaller than that of RbNbB 2 O 6 , and KNbB 2 O 6 is not phase-matchable at this wavelength. Nevertheless, it is possible to tune the birefringence of KNbB 2 O 6 by replacing some K + ions by the larger Rb + ions so that non-critical phase matching may be possible at room temperature.