Cesium lithium borate single crystals tend to crack at room temperature in an ambient air atmosphere, which has limited their use in device applications. This study shows that the cracking of high-quality CLBO crystals results from the anisotropic corrosive effect of water vapor in the ambient atmosphere. The directional attack by water molecules at the crystal surface was related to its crystal structure. The crystal quality of CLBO was found to influence the crack propagation velocity. (C) 2002 Elsevier Science B.V. All rights reserved.