In this study, the sorption behaviour and conductivity of two anion exchange membranes (AR103 and AR204) equilibrated with sodium acetate and sodium lactate solutions are studied across a range of concentrations and pH values. The results indicate that the dissociation equilibria of the organic acids differ between the membrane phase and the external solution. There are significant concentrations of the neutral organic acid in the membranes at pH 6.5 even though the dissociation is virtually complete in the external solution. The concentration of this neutral acid increases as the pH is lowered, leading to a reduction in membrane conductivity. The diffusion coefficients of acetate and lactate ions in these membranes are determined from conductivity data. The results show that these diffusion coefficients are relatively constant but decrease slightly with an increase of external solution concentration due to osmotic deswelling. The diffusion coefficient of the acetate anion decreases as the pH falls, possibly due to dimer formation. Models extended from Manning's condensation theory have been utilized in the prediction of the co-ion concentrations within the membrane and the diffusion coefficients of the lactate and acetate anions. There is an excellent agreement between the experimental values of these parameters and the model predictions for the sodium lactate system but the model is unable to accurately fit the sodium acetate data.