Ion-selective membranes are widely used as sensing materials in many diagnostic fields such as the pharmaceutical, environmental, and industrial fields. Ion-selective membranes are normally constructed from a neutral polymer such as polyvinyl chloride and allow ion transportation to occur through the crown structure of ionophores. To characterize the functionality of the membranes after prolonged exposure to solutions of various interfering ions, electrochemical impedance spectroscopy is utilized to investigate membrane performance, and the results are compared with those obtained from atomic force microscopy. In this study, the pore size of a membrane is quantified by using a pore size model and compared with the separation between adjacent corrugations in the profile line obtained from atomic force spectroscopy images by using image processing software.