The study of localized corrosion is hindered by the inability to characterize electrolyte compositions inside crevices and pits during the corrosion process. This paper describes the development of a microelectrode system for in situ monitoring of the pH in artificial crevices and small recesses. This system permits determination of the pH gradient inside a crevice, even in the presence of a pronounced potential drop. The pH sensor, consisting of a palladium hydride electrode, can be positioned at any desired depth into the crevice while observing the position of the passive to active transition. With increasing time of crevice corrosion, one finds a pronounced pH change in the crevice electrolyte. This work lends further insight toward understanding of crevice corrosion and other localized corrosion phenomena. (C) 2005 The Electrochemical Society. All rights reserved.