[en] We have studied phase relations in the pseudoquaternary system LuAl₃(BO₃)₄–(K₂Mo₃O₁₀–Al₂O₃–B₂O₃) in the temperature range 1130 to 900°C. The phase relations were represented in graphical form, as a projection of the formation of solid phases in this range onto the composition triangle at 900°C. Optimal results in terms of high-temperature solution growth have been obtained at 25 wt % LuAl₃(BO₃)₄ in the growth charge. Undoped and erbium–ytterbium codoped LuAl₃(BO₃)₄ (LuAB) single crystals have been grown from high-temperature solutions on “pointlike” seeds. The (Er,Yb):LuAB crystals are similar in thermal properties and luminescence spectra to (Er,Yb):YAl₃(BO₃)₄ and (Er,Yb):GdAl₃(BO₃)₄ crystals and can be used in diode-pumped lasers operating in the nominally eye-safe spectral region 1.5–1.6 μm.