The interstitial concentration and temperature dependence of the diffusion coefficient ratio D12/D11 in ternary austenites containing silicon or manganese has been experimentally determined using diffusion couples based on the concept of "transient equilibrium". The results are compared with values predicted theoretically from statistical and thermodynamic considerations. </p> <p> The kinetics of the growth of proeutectoid ferrite in binary and ternary austenites have been determined, using two-phase diffusion couples. The results are consistent with the premise that carbon diffusion controls the rate of interface migration. It is demonstrated, theoretically and experimentally, that the ternary results fall into two classes - one in which local equilibrium may be assumed, and one in which "constrained local equilibrium" may be postulated. / Thesis / Doctor of Philosophy (PhD)