Infrared has been used in determination of water in magnesium metaphosphate glasses. The method consisted of measuring the weight loss and the variation in absorption coefficient at 3, 200$cm^{-1}$. The molar absorptivity of water in magnesium metaphophate glasses was found to 76.9ι$mol^{-1} cm^{-1}$ Using the value for $varepsilon$, A and Δ$upsilon$H of the hydroxyl band in the spectra of magnesium metaphosphate. The molar absorptivity of the hydroxyl absorption band at 3, 3130-2, $925cm^{-1}$ in the spectra of MgO.$P_2O_5$ system glasses could be computed. And a variation of molar absorptivity against the MgO/P2O5 ratio has not been shown regulation.