An electrochemical microcell was developed for potentiometric pH measurements of supercritical aqueous solutions. It has an internal volume of ca. 1.5 cm 3 and a surface area of an electrode of 0.04 cm 3 These values are around ten times lower than that of previous apparatus [K. Sue, M. Uchida, T. Usami, T. Adschiri, K. Arai, Apparatus for direct pH measurement of supercritical aqueous solutions, J. Supercrit. Fluids 28 (2 - 3)(2004) 287]. The decrease in residence time in the cell allows more reliable pH measurements of thermally unstable compounds such as organic acids. With the newly-designed cell, pH measurements were carried out for HCI + NaCl aqueous solutions at flow rates from 1.0 to 4.0 g/min, temperatures from 21.5 to 390.1 degrees C and pressures from 22.0 to 30.1 MPa. Despite the large decrease in surface area of the electrodes, the experimentally determined pH showed good agreements with the theoretically calculated pH over the range of conditions studied. (c) 2006 Elsevier B.V. All rights reserved.