An in situ surface study of the iron chalcogenide glass membrane ion-selective electrode (ISE) in aqueous media has been undertaken using a tandem technique of mixed potential/synchrotron radiation grazing incidence X-ray diffraction (SR-GIXRD) and atomic force microscopy (AFM). This work has simultaneously monitored the mixed potential and in situ surface diffraction patterns of this crystalline glassy material, showing that the observed gradual shift of the electrode potential in the anodic direction is linked to the preferential dissolution of the GeSe (111), GeSe (101) and GeSe (141) and/or Sb2Se3 (0 13), Sb2Se3 (2 2 1) and Sb2Se3 (0 2 0) surfaces. Expectedly, these observations are internally consistent with preferential oxidative attack of the crystalline regions of the membrane comprising GeSe and/or Sb2Se3, as evidenced by AFM imaging of the electrode surface. Clearly, this work corroborates the results of previous ex situ surface studies on the iron chalcogenide glass ISE, whereby it was shown that alkaline saline solutions have a tendency to alter the surface chemistry and concomitant response characteristics of the ISE. (c) 2006 Elsevier Ltd. All rights reserved.