Barium and potassium hydroxide have been investigated as fluxes for the growth of cuprate single crystals. The relatively high solubility of transition metals and lanthanoids in these salt fluxes at moderate temperatures allows significant lowering of the growth temperatures required for many phases. Also, phases not stable at high temperatures become accessible. Two new cuprates have been prepared in the Ba-Ca-Cu-O and Ba-Y-Cu-O systems from a Ba(OH)2 · H2O flux. The compounds Ba3(Y0.23Cu0.77)2O5.78 and Ba3(Ca0.24Cu0.76)2O4.43 crystallize in a tetragonal (space group I4/mmm) oxygen deficient Sr3Ti2O7-type structure with lattice parameters a = 4.069(2) Å, 4.022(1) Å and c = 21.61(2) Å, 21.63(2) Å, respectively. The compound (Ba0.92Sr0.08)(Ca0.38Cu0.62)O 2.1 crystallizes with a doubled perovskite unit cell along all three axes, a = 8.116(4) Å. In addition, single crystals of Ba2YCu3O7-δ have been prepared from a KOH flux at 750 °C.