We propose a multi-component species transport benchmark which combines diffusion and fluid flow. By using identical species input volume rates per minute into a test geometry and in absence of chemical reactions, the species mass fractions are known analytically exact after complete mixing, which permits to evaluate the absolute numerical error of the diffusion computation. it is demonstrated that the absolute error of the mass fractions is reduced quadratically with decreasing the element size of the grid. The finite-element-method is applied for solving the system of equations. (C) 2009 Elsevier Ltd. All rights reserved.