Abstract: A reaction vessel for conducting a chemical or biochemical reaction, such as a polymerase chain reaction wherein electrically conducting polymer is arranged to act as a heating element. The profile of the electrically conductive polymer differs in different regions of the vessel so as to control thermal gradients. The profile of the electrically conductive polymer may be arranged to either increase or reduce the thermal gradient. Reaction systems comprising combinations of vessels of the invention and apparatus for heating them, as well as particular reactions vessels are also described and claimed.