An apparatus is described which extends the time range of the temperature‐jump technique at high pressures to the nanosecond region. A temperature increase of a few degrees can be obtained within approximately 50 ns by discharging a high‐voltage coaxial cable. Reactions are followed spectrophotometrically. Details of the temperature‐jump microcell and high‐pressure autoclave are presented. An application of this method is given for the reaction of phenol red with NH+4/NH3buffer at 1 and 2000 bars.