The influence of pyroxene on the rheology of peridotite is critical for understanding the dynamics of the upper mantle. Intragranular deformation of each phase is often assumed to be independent and the rheology of aggregates is calculated by mixing models. However, intergranular deformation on grain boundaries (e.g., olivine-olivine) and phase boundaries (e.g., olivine-pyroxene) could be intrinsically different and needs to be constrained experimentally. Experiments on fine-grained olivine and clinopyroxene demonstrate that 50-50 mixtures of these phases (a wehrlite) deform up to ∼30 times faster than either of the end-members when scaled to the same experimental conditions. Strain localization within layers with a high-density of phase boundaries demonstrates that phase boundaries enhance the rate of deformation relative to that of either olivine or pyroxene aggregates. Olivine AG-type crystallographic textures (i.e., axial girdles of  and  axes with  maxima sub-perpendicular to the shear plane) observed in deformed specimens are indistinguishable from those observed in many naturally deformed mantle peridotites. We propose that the presence of abundant olivine-pyroxene phase boundaries can decrease the viscosity of mantle shear zones, enhancing strain localization.