The effect of amino acids at zwitterionic condition on the water uptake of Aerosol-OT (AOT) reverse micellar system has been studied. The experimental results for different initial surfactant and salt concentrations, and different amino acids showed that the water uptake increased almost linearly with the initial amino acid concentration. The variation of the water uptake with the amino acid concentration is purported to be due to the counterion effect of amino acids. A simple model is proposed to account for the counterion effect of amino acids. The model accurately predicts the water uptake by the reverse micelles.