In general terms the biosensor apparatus according to the invention comprises a housing (5, 6), a pair of heat sinks (11, 12) and a pair of heat reflectors (13, 14) thermally floating relative to the heat sinks, and a pair of Peltier elements (16) attached to and in thermal contact with the heat sinks (11, 12), one element (16) on each heat sink (11, 12). The housing is made of an insulating material, and the heat sinks (11, 12) are made of a material with thermal properties large enough to absorb the heat flow with only a very slight, negligible disturbance in its temperature, to render their envelope surface having a high heat emissivity. The heat reflectors (13, 14) are made of a material having a very low emissivity and have a generally flat and thin disc shaped structure and acts primarily as radiation shields.