A new methodology, based on the numerical resolution of the nondimensional momentum and mass transport equations, is applied here in order to analyze the impact of free surfaces on the measurement of the diffusion coefficients using shear cells at high temperatures. If solutal Marangoni convection acts during all the process, calculations suggest that the percentage deviation of the measured coefficient quadratically scales with the solutal Marangoni number. Also, the orientation between the gradient of concentration and the direction of the residual gravity appreciably influences the accuracy of the values, independently on the initial free surface extension. In the cases in which, by oxidation or reduction, the extension of the free surfaces decreases or increases as a function of time, the percentage deviation seems to increase also in a quadratic way as a function of the characteristic time, an indicator of the period of activity of the time dependent Marangoni convection. The orientation between the gravity vector and the gradient of concentration maintains the same tendency in both cases. (C) 2014 Elsevier Ltd. All rights reserved.