Mass convective flow was quantified by direct measurements of velocity and concentration fields of the solute in high temperature solution near the interface using an optical in situ observation technique. All experimental facts can be described by calculation of the velocities of normal and transversal components of solute diffusion-induced bulk flow. A simpler analytical mass boundary layer equation was deduced to identify the dominant convective flow. The measured velocities are understood as interchange-ability of diffusion-induced bulk flow and concentration boundary layer model. (c) 2006 Elsevier B.V. All rights reserved.