We describe the preparation of [(100 - gamma)/(4)](SrO-BaO-2Nb(2)O(5))-gamma P2O5 (SBNP gamma) glasses and glass-ceramics with the Sr0.5Ba0.5Nb2O6 (SBN50) phase, using a conventional process, and the analysis of the chemical stability of the materials in water. The SBNP gamma glasses and glass-ceramics consist of orthophosphate (Q(0)) and pyrophosphate (Q(1)) units with strongly delocalized P=O bonds. Because of the presence of these phosphate units, the glasses and glass-ceramics obtained in this study show no weight changes, even after exposure to water at 70 degrees C for 100 h. We hypothesize that the lack of phosphate chains within the glass network is responsible for the superior water stability.