Piezodialysis in weakly amphoteric polymer membranes is discussed from the viewpoint of non-equilibrium thermodynamics. Succinyl chitosans were used as the weak polyampholytes. With varying pH in the outer solution, the apparent isoelectric point of the membrane was defined as the point of zero membrane potential, where solute permeation is much enhanced. Piezodialysis was confirmed from the phenomenological hydraulic permeability coefficients, reflection coefficients and solute permeability coefficients of the membrane, which were based on non-equilibrium thermodynamics. Permeation experiments with mixed ion systems were also performed. The permeation behaviour was in accordance with that expected for the interactions of ions with polyampholytes in aqueous solutions.