Abstract: A chemiluminescence detector is comprised of a gaseous reaction chamber having a light transmitting element mounted to define one wall of the reaction space. A light responsive device, for example a photomultiplier, is arranged to receive light generated by a chemiluminescent reaction within the reaction chamber and transmitted through the light transmitting element. A thermoelectric heat pump means is situated in proximity to the reaction chamber and to the photomultiplier. The heat pump means is arranged to provide for cooling of the photomultiplier by extracting heat therefrom while simultaneously heating the reaction chamber. The thermoelectric heat pump is comprised of at least one mounting gasket means arranged to have a first surface in contact with the housing of the reaction chamber and a second surface in contact with a portion of the housing of the light sensing means.