Analytic approximate expressions are derived for the electrostatic interaction between two dissimilar membranes immersed in an electrolyte solution. The membrane-fixed charges are assumed to be uniformly distributed through an ion-penetrable surface layer on the membrane core. The effects of the ionic solubility, the relative permittivity and the thickness of the surface charge layer are taken into account. The membrane surface potential arises both from the membrane-fixed charges and from the difference in partition coefficient between cations and anions in the surface charge layers. It is predicted that there appear six types of interaction force (and thus the potential energy) versus membrane separation curve. Our model is found to include, as its limiting forms, two different conventional models for the electrostatic interaction: (1) the constant surface potential model and (2) the constant surface charge density model. In the limit of large ionic partition coefficients, our model reduces to model (1); while, on the other hand, in the limit of zero thickness of the surface charge layers our model becomes indentical with model (2).