The apparatus for electrochemical measurements in high-temperature water has been improved by replacing the alumina tube reactor with an oxidized titanium tube, producing a more rugged reactor. All metal fittings were replaced by titanium ones, decreasing corrosion problems. The working electrodes are Pt disks (radius, 14 mu m) fabricated by sealing Pt-Ir wires in a high PbO-containing glass. This glass has the advantage over borosilicate glass in showing lower conductivity at temperatures above 230 degrees C, better corrosion resistance, and a coefficient of thermal expansion well-matched to Pt. Well-defined steady-state cyclic voltammograms for the oxidation of iodide ion and hydroquinone were obtained in (initially 23 degrees C) 0.2 M NaHSO4 solutions over a temperature range 23-385 degrees C. Diffusion coefficients (D) were determined from the limiting currents. The experimental D values were in excellent agreement with those calculated from the Stokes-Einstein equation and viscosity values for water at the relevant temperatures and pressures.