Cs-Li borate crystals (CLBO) grown from melts have been examined by chemical and thermal analysis and IR spectroscopy. Evaporation of cesium oxides from melt during growing resulted in crystals with Cs deficit. The defect structure of these crystals has been refined by means of single-crystal X-ray diffraction analysis, while two non equivalent Cs positions have been revealed. Using the close-packed spheres model, the volumes of the structural vacant spaces have been estimated, and the proof of their occupation by one or several water molecules has been given. The behavior of the defect CLBO crystals exposed to dry and moist atmospheres has been considered. (c) 2005 Elsevier B.V. All rights reserved.