The deformation behavior of synthetic dunite (Mg-forsterite plus 2.5 vol% enstatite) with a grain size of ~1 μm has been investigated in combined constant displacement rate and temperature stepping experiments at temperatures of 850¿ to 1000 ¿C, strain rates of 10-7 to 10-5 s-1 and a confining pressure of 600 MPa. Nominally dry material behaved purely elastically. In contrast, samples with 0.5 wt% added water flowed at stresses between 9 and 81 MPa reaching strains up to 12%. Fitting a Dorn-type power law to the wet data yielded stress exponents (n) of 1.7¿0.4 and apparent activation energies in the range 302¿22 kJ mol-1. The wet-deformed samples showed polygonal grains, no crystallographic preferred orientation, no subgrains, low dislocation densities, and evidence for grain boundary cavitation. Grain growth was minor. It is concluded that the wet samples probably deformed by a grain size sensitive, grain boundary sliding dominated mechanism, though the nature of the accommodation process remains unclear, and the possibility of transitional behavior between dislocation creep and diffusion creep cannot be eliminated. Application of the results to upper mantle shear zones supports previous speculation that the presence of such zones can lead to a significant weakening of the top 10 to 20 km of the upper mantle during extension of the continental lithosphere. ¿ 1999 American Geophysical Union