Glasses with 55–60mol% SnO and 40–45mol% P 2 O 5 have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550°C, had low T g , 150–200°C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200°C, had much higher T g , 250–300°C, and much higher chemical stability. No significant differences were found by 119 Sn Mössbauer and 31 P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and 1 H Nuclear Magnetic Resonance spectroscopy. In samples with low T g , a broad OH – vibration band around 3000nm with an absorption intensity >20cm −1 , bands at 2140nm with intensity ∼5cm −1 , at 2038nm with intensity ∼2.7cm −1 , and at 1564nm with intensity ∼0.4cm −1 were measured. These samples have shown a mass loss of 3–4wt% by thermal gravimetric analyses under argon in the temperature range 400–1000°C. No mass loss and only one broad OH-band with a maximum at 3150nm and low absorption intensity <4cm −1 could be detected in samples melted at high temperature, 1000–1200°C, which have much higher T g , ∼300°C, and much higher chemical stability.