This work describes the determination of carboxylic acid overall mass transfer coefficients in a circulating flow experimental set-up composed of a flat plate dialyzer and a concentrate reservoir. Sodium acetate and sodium propionate were used as the sodium salts of carboxylic acids. By referring to the model generated concentrate reservoir concentration vs. time data, we focused our attention on the quantitative justification of the previously published equation of ours that could be implemented as a new tool in the determination of overall mass transfer coefficients in dialysis processes. Furthermore, the present study was undertaken for the comparison of unsteady state case, i.e. rigorous solution, predicted overall mass transfer coefficients against a simpler expression based on a pseudo steady state approximation. Contrary to the concentrate side, single pass flow was used on the diffusate side where the receiving liquid was deionized water. It Was found that the overall mass transfer coefficient values, K for both solutes increased with the increase of the flow rate. The K values calculated by rigorous solution, i.e. taking into account the unsteady state effects, were always higher than those obtained via pseudo steady state approximation.