The deposition of polyaniline (PANI) onto the surface of plasticized poly(vinyl chloride) (PVC) membranes requires optimizing the membrane composition. Controlling the ratio between the plasticizer and PVC enables the hydrophilic surface to be prepared and a homogeneous PANI film to be deposited through chemical polymerization. Our studies showed that such modified PVC membranes can be used for the potentiometric detection of three tumor markers (TMs) that are metabolites of catecholamines and serves as a screening test for neuroblastoma. Furthermore, the interactions of PANI with these TMs were proved by a capillary electrophoresis method in which a capillary covered with PANI was used.