A study on the electrolysis of sub- and supercritical aqueous solutions of sodium hydroxide is presented here. Current density-potential curves are recorded at temperatures up to about 800 K and pressures up to 400 MPa. The molar concentration of the solutions, as referred to T = 298 K, is varied from c = (0.01 to 1) mol(.)dm(-3). The autoclave is described in detail. Several electrode materials are examined. The highest reproducibility is obtained with gold electrodes. Some results are also given for platinum, silver and nickel electrodes. Below T = 473 K the current-potential curves show the familiar transition from a range of very low currents to an ohmic behaviour with steeply increasing currents above the decomposition potential. The decomposition potential decreases with increasing temperature. Above T = 473 K the current-potential curves no longer reflect distinct transitions from low-current to high-current behaviour, and at T = 773 K, almost linear current-potential curves are observed over the entire voltage range. Up to current densities of 50 mA(.)cm(-2), the pressure dependence of the current density at a given potential is marginal. At higher current densities, up to 35 A(.)cm(-2), a substantial pressure effect is observed. (C) 2003 Elsevier Science Ltd. All rights reserved.