Melt vicosities and densities of isobarically quenched melts (glasses) have been determined for K2Si2O9 up to pressures of 2.4 GPa. At 1200¿C, melt viscosity decreases linearly from 1700 P at 1 atm to 310 P at 2.4 GPa. The increase in glass density with increasing pressure is described by a Birch-Murnaghan equation of state with values of the 1-atm bulk modulus and its pressure derivative equal to 32.5 GPa and 7.6, respectively. The structure of the quenched melts was investigated using Raman spectroscopy. Scattering in the 900--1000-cm-1 region increases in intensity with increasing pressure, and a new band develops at 940 cm-1, suggesting an increase in Q2 species, perhaps accompanied by the presence of Si in high coordination. In the low-frequency region the 520-cm-1 band of the 1-atm glass increases slightly in frequency with pressure, and a distinct shoulder at 470 cm-1 develops on this band. These observations suggest an increase in the number of Q4 species and a narrowing of the SiOSi bond angle within Q3 groups in the glass. The structural changes are consitent with a shift in the silicate species disproportionation equilibrium, 2Q3=Q4+Q2, to the right with increasing pressure, and the changes provide a mechanism for explaining the observed variations in viscosity and glass density. ¿American Geophysical Union 1990