AbstractIonic polymer‐metal composites (IPMC) are promising materials in biomedical actuators and sensors, as they are soft and flexible, thus assuring the safety of the device itself. The purpose of this study is to investigate the biocompatibility of IPMC in in vitro experiments in order to evaluate their applicability in biomedical fields. In addition to an IPMC specimen prepared by the conventional “impregnation‐reduction method” using cationic gold complexes and reducing agents, two specimens were prepared by additional processes: one specimen was reduced in Na2SO3 solution and the other was cleaned in H2O2 solution. A colony‐forming test using Chinese hamster V79 cells showed high cytotoxicity for all IPMC specimens. Examination by direct inlet mass spectrometry (DI‐MS) revealed that the peak intensity of the gold complex (particularly, m/z=180) was different from that of Nafion film. Monitoring the peak at m/z=180 showed a remnant with the structure of phenanthroline in IPMC specimens that were not cleaned in H2O2 solution. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 93(1): 1–8, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10239