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The temperature dependence of the diffusion coefficient for carbon diffusion in a fcc Fe-29.89 w/o Ni Invar, D, was obtained over 8 decades from various relaxation-time measurements. Combined with published high temperature data, (1) diffusivities over 14 decades of D are available for discussion. The resulting Arrhenius plot is distinctly non-linear and the deviation from linearity is large and comparable in magnitude to that observed for self diffusion in Beta-Ti. An empirical expression for the temperature dependence of D modeled in terms of Flynn's expression for interstitial diffusion is provided. It satisfactorily describes the data and shows that the deviation from linearity arises predominately from a decrease of the lattice strain energy Es. Es has strain components comparable to that arising from the Bain distortion and by way of analogy, it is suggested that the large diffusion anomaly observed for carbon interstitial diffusion in this alloy is a precursor to the martensitic transformation. (DePo)