An apparatus 20 for biological or chemical reactions, in particular PCR, includes a heat removal module 22 adapted to receive snugly a reaction vessel 24 in such a manner as to create good thermal conductivity contact between the module and the vessel. The heat removal module 22 is formed of a thermally conductive material having therein a channel 64 adapted for the flow of a coolant liquid. The heat removal module 22 is constructed with an array of receiving stations 62 for the reception of a corresponding array of reaction vessels 24.