This study dealt with diffusion dialysis process of single HCl solution as feed. Water osmosis phenomenon was investigated in detail and modified mass transfer equation with the chemical potential difference as driving force was deduced. It mainly included three parts. Firstly, time dependence of diffusate volume was investigated quantitatively, and when the curves were divided into two parts and two different quadratic polynomials were used to fit them, results showed that the fitting accuracy was satisfactory. Secondly, to calculate the solution chemical potential more easily, average mean activity coefficient and average osmotic coefficient were used. Results showed that with the increase in feed concentration, the coefficients increase accordingly. Thirdly, permeability coefficients were investigated to quantify the diffusion dialysis, results illustrated that when lambda=5 and theta=0.5, P'(HCl) and P'(H2O) were acceptable coefficients to characterize the process. This study can allow a selection of optimum parameters for diffusion dialysis process. (C) 2014 Elsevier Ltd. All rights reserved.