The effect of varying enstatite content on the deformation behavior of synthetic, fine grained (1 to 2 μm) forsterite-enstatite rock with ~0.5 wt % added water was investigated at temperatures of 900¿ to 1000 ¿C, strain rates between 10-7 and 10-5 s-1, and a confining pressure of ~600 MPa. The samples exhibited approached steady state flow at stresses ≤60 MPa. The results show that, at constant strain rate, increasing enstatite content is associated with a sharp decrease in flow strength in the range 0--2.5% vol % enstatite, with little further change up to 20 vol %. The observed power law n value of ~1.7 and microstructures are similar to those obtained in previous work on material with 2.5 wt % enstatite and are consistent with a water-enhanced grain boundary sliding (GBS) dominated deformation mechanism. Significantly, a negative correlation was found between grain size and enstatite content, indicating that enstatite content played a role in controlling the grain size of the starting materials. Moreover, a high correlation between measured flow strength and grain size was found, consistent with a grain size exponent of -3 in a conventional grain size sensitive flow equation. A water-enhanced deformation mechanism involving GBS accommodated probably by grain boundary diffusion and/or dislocation activity is therefore implied, with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control. Other effects of second-phase content, such as weakening caused by interphase boundary diffusion and/or migration processes, seem to be unimportant under the conditions investigated. ¿ 2000 American Geophysical Union