The common diffusion process is always understood with the passive movement of particles from a high concentration area to a low concentration (forward diffusion). In this paper, another diffusion process (backward diffusion process) is presented. In this diffusion type, the diffusion flux is along with the direction of increasing concentration. Furthermore, the kinetics, the thermodynamics and the equation of the backward diffusion are studied and discussed. The results have showed that: although the backward diffusion process is opposition to fundamental diffusion laws (Fick’s laws), this process is not contradictory with the basic principles of thermodynamics; the backward diffusion can be described by equations that are similar to the Fick’s equations (with negative diffusion coefficient); the analytic solutions of the backward diffusion equation can be found; and the divergence of general solution of the backward diffusion equations is also solved.

backward diffusion, negative diffusivity.