In this contribution, we theoretically investigate the isothermal mass transport of a simple fluid into a blend of two immiscible Newtonian polymers. Using internal state variables, we derive a nonlinear formulation that addresses the effects of the diffusion/interface coupling on both the mass transport as well as on the morphology of the interface. The approach uses a scalar and a second-order tensor to directly track the dynamic changes of the size and shape of the interface. The mass flux governing equation includes new terms that lead to non-Fickian behavior attributed to the viscoelatic contribution of the interface. In turn, the size and shape of the interface are modified by diffusion. In one-dimensional analysis, we examine the nature of propagation of both nonlinear hyperbolic and linear dispersive waves. Explicit formulas for the characteristic speed, phase velocity, and attenuation are provided. (C) 2003 American Institute of Physics.