The process of m1x1ng two liquids to the molecular level is extremely complex, even under conditions of laminar motion, and takes on additional complexity when the liquids react chemically. As a consequence the relatively simple example treated here has the advantage of providing a transparent, but incisive, description of the processes that are essential in more complex situations. We consider a two-dimensional motion in which, initially, two different liquids are situated respectively in the upper and lower half-planes. At the start of motion a vortex is imposed at the origin. The interface between the two liquids, across which molecular diffusion is taking place, is distorted and stretched in a manner resembling a conventional stirring process in liquids.