Diffusion along  is faster than diffusion along  by a factor of 3.5, while diffusion along  is similar to that along . Comparison of DFe-Mg and rates of order-disorder kinetics indicates that for around the IW buffer and lower, diffusion in natural orthopyroxene becomes insensitive to , which could be related to a transition in the diffusion mechanism from a transition metal extrinsic (TaMED) domain to a pure extrinsic (PED) domain. This behavior is analogous to that observed for Fe-Mg diffusion in olivine and this complexity precludes the formulation of a closed form expression for the composition and dependence of DFe-Mg in orthopyroxene at present. We were not able to quantitatively constrain the dependence of DFe-Mg on the XFs content from the profile shapes, but consideration of the experimentally measured diffusion coefficients along with the data for order-disorder kinetics suggests that the compositional dependence is weaker than previously estimated, at least for orthopyroxene with XFe < 0.5. For the major element compositional T and range of available experimental data, Fe-Mg interdiffusion in orthopyroxene is slower than in olivine and aluminous spinel, comparable to garnet, and faster than in clinopyroxene.