In quasibinary systems with a spinodal miscibility gap interdiffusion experiments above and below the critical temperature of demixing were carried out. The influence of thermodynamics on the Arrhenius parameters for interdiffusion (D) over tilde (0,A)(x) and E-A(x) is described in dependence on the excess Gibbs energy, which can be calculated, if the thermodynamical interaction parameters of the system are known.The interdiffusion coefficient is composed of the mobility term and the thermodynamic factor. With the assumption that the logarithm of the mobility term depends linearly on composition the evolution of the thermodynamic factor is extracted from our experimental diffusion data and compared to the predictions obtained with our theoretical model. It is shown that in all systems with remarkable deviation from ideal behavior the interdiffusion coefficient yields a minimum at the critical composition of demixing and the Arrhenius parameters in dependence on composition show maxima that tend to infinity by approaching the critical point.The so-called correlation effect in the composition dependence of the Arrhenius parameters of interdiffusion can qualitatively be understood in systems that show a distinct deviation from ideal behavior. (C) 2003 Elsevier B.V All rights reserved.