To better understand the kinetics associated with the uphill diffusion of carbon (C) in the austenite (γ) phase of low-carbon steels, the effect of substitutional component M on the intrinsic diffusion coefficient, DCγ, of C in the γ phase of the ternary Fe–C–M system was quantitatively analyzed based on thermodynamics. Here, M corresponds to various metals, including Mn and Si. When the concentration, cMγ, of M in the γ phase is homogeneous, this effect was found to be negligible. In contrast, the effect was not negligible in the case of C diffusion trapped by M. If cMγ is inhomogeneous, DCγ varies depending on the concentration, ciγ (i = C and M), in the γ phase and the ratio of the gradient of cCγ to that of cMγ. In this case, DCγ takes positive or negative values, and the negative value results in the uphill diffusion of C. Using the (Fe–C)/(Fe–C–Si) diffusion couple as an example, the diffusional flux, JCγ, continuously varies with the distance, r, along the direction normal to the original interface, although the dependence of cCγ on r is irregular across the original interface. The extent of uphill diffusion can be estimated from the diagonal and off-diagonal diffusion coefficients for the intrinsic diffusion of C and from the value of ciγ.