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Viscosities of synthetically hydrated obsidians have been measured by the falling‐sphere technique at two values of the H2O content. At 4.3 per cent H2O, by weight, the common logarithm of the viscosity in poises ranges from 6.51 at 800°C to 5.88 at 900°C, and at 6.2 per cent H2O from 6.54 at 700°C to 5.25 at 850°C. The effect of pressure on the viscosity is shown experimentally to be slight at these temperatures. The data are consistent with viscosity isotherms determined by Friedman, Long, and Smith on rhyolite glass at H2O contents to 1.25 per cent. Activation energies and pre‐exponential constants determined from this work and that of Friedman et al. show striking similarities to data for binary alkali oxide‐silica systems. This suggests that the discrete ion theory of silicate melts proposed by Bockris and co‐workers is applicable to silicate‐H2O systems. On the basis of this model a generalized graph is presented which permits approximate predictions of viscosities of framework silicate‐H2O systems for H2O contents up to about 10 per cent by weight and temperatures in the range 500° to 1200°C.