The disaccommodation technique was used to determine the activation volume of carbon diffusion, $\ensuremath{\Delta}V$, in six fcc iron-nickel alloys. Measurements were made at temperatures ranging from 68 to 90 \ifmmode^\circ\else\textdegree\fi{}C and pressures up to 6 kbar on samples having a nickel content between 31 and 63 at.%. A maximum activation volume of 3.9 ${\mathrm{cm}}^{3}$/mole was found at 34 at.% nickel. The compositional dependence of $\ensuremath{\Delta}V$ is satisfactorily reproduced by a magnetic-energy continuum model in which it is assumed that the activation free energy of diffusion is essentially ferromagnetic in origin.