The conditions of HNbO3 precipitation on the lithium niobate single-crystal and titanium diffused lithium niobate, LiNbO3 (LN) were studied. It has been shown, that the maximum of HNbO3 precipitation corresponds to the highest humidity of atmosphere at the annealing temperatures of 800-850 degreesC for 5-10 h. Moreover, the application of humid atmosphere during the annealing of LN at 600-850 degreesC does not ensure in full measure the lack of LiNb3O8 precipitation. (C) 2003 Elsevier B.V. All rights reserved.