The grain-scale processes of peridotite partial melting were examined at 1340¿C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized harzburgite. Reaction between clinopyroxenite and harzburgite produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Clinopyroxene compositions vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P-T conditions. The growth of the reactive boundary layer is at the expense of the harzburgite and is consistent with grain-scale processes that involve dissolution, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals.