Abstract: There is described a variable-temperature reactor for hosting a predetermined reaction therein. The reactor comprises a reaction cell, a heater, and a heat sink. The reaction cell has a reaction volume with thickness Hv and width Wv where Wv>4Hv and is defined by faces with one of the larger area faces of the reaction volume being bounded by an outer wall with thickness Hw. The heater is in contact with the said outer wall. The heater comprises a heat-generating heater element located on the face closer to the reaction volume and a heater support on the opposite face. The heater support is in contact with a heat sink, such that the heater support provides a thermal resistance RT between the heater element and the heat sink. The reactor, when filled with reagents having thermal diffusion coefficient Dv has a diffusion time tv, in the thickness direction, tv=Hv2/Dv. tv is less than the reaction time constant tR.