A part of the Cs2O-B2O3 system was studied in an attempt to determine a self-flux composition suitable for the growth of CsB3O5 (CBO) crystals. The phase diagram in the region near the composition of CBO, viscosity, and volatility were investigated in order to gain a better understanding of the properties of self-flux solutions. We then tried to grow CBO single crystals from solutions having various compositions. A Cs2O-rich solution close to stoichiometric composition (75 mol% B2O3) enables crystals to grow stably at a high growth rate. However. crystals grown from a composition higher than 72 mol% B2O3 contain numerous scattering centers. These results indicate that a careful optimization of the starting composition of the self-flux solution is necessary in order to obtain high-quality, CBO crystals. (C) 2004 Elsevier B.V. All rights reserved.