Nitrided glasses with the composition 25R(2)O.20MO.55P(2)O(5) (R = Li, Na, K, M = Ba, Pb) were prepared by treatment in an anhydrous ammonia atmosphere between 650 and 800 degrees C for periods of 3 to 40 h. The nitrogen content of the glasses increases linearly with the treatment temperature; as a function of time, the N-2 content tends toward an upper limit, which depends on the temperature. The nitrogen introduction into these glasses causes an increase in the transformation and softening temperatures, microhardness and toughness, while the expansion coefficient decreases slightly with the nitrogen content. However, the rise of the hydrolytic resistance of the glasses by near two orders of magnitude is the most important change caused by nitrogen introduction. The structural study by IR spectroscopic and the analysis of the glass weight losses during the thermal treatment reveal that nitrogen replaces the bridging oxygens in the phosphate network.