Consideration of the ternary phase relationships between components of the XYB 2 O 6 borates, where X=Na, K, Rb, or Cs and Y=Nb or Ta, suggest that the conditions for their optimal single-crystal growth can be determined from predicted pseudo-binary phase diagrams. Several compounds have been synthesised on the basis of the predicted Nb 2 O 5 : Rb(K) 2 B 4 O 7 pseudo-binary systems and single crystals of KNbB 2 O 6 (KNB) and RbNbB 2 O 6 (RNB) grown for the first time by spontaneous nucleation from their self-fluxes. Larger crystals of RNB were grown and these also had a better optical quality. Small crystals of alkali-substituted KNB (Cs 0 . 3 K 0 . 7 NB) and RNB (Na 0 . 0 7 Rb 0 . 9 3 NB and K 0 . 4 4 Rb 0 . 5 6 NB) were also grown by spontaneous nucleation to provide additional information on the structure-property relationships of the XYB 2 O 6 compounds. Single crystals of both KNB and RNB measuring ~20x10x10mm 3 were grown by top-seeded solution growth (TSSG) using the same melt compositions as used in spontaneous nucleation. These crystals contained large densities of bubbles and solvent inclusions. However, small (5x5x3mm 3 ) defect-free samples could be cleaved from these crystals. Even larger single crystals (~38x33x6cm 3 ) of RNB were grown by TSSG from an Rb 2 MoO 4 flux and these were free of inclusions. The potential of these single crystals for application in nonlinear optical devices was assessed by X-ray diffraction, optical absorption, laser damage threshold and efficiency of second harmonic generation at 532nm.