Abstract: Temperature control in a rectangular array of reaction vessels such as a thermal cycler such as is used for PCR procedures is achieved by use of a temperature block that is in contact with a combination of Peltier effect thermoelectric modules and wire heating elements embedded along the edges of the block. The elements can be energized in such a manner as to achieve a constant temperature throughout the array or a temperature gradient. Further control over the temperature and prevention of condensation in the individual reaction vessels is achieved by the use of a glass (or other transparent material) plate positioned above the vessels, with an electrically conductive coating on the upper surface of the glass plate to provide resistance heating.