Cyclic fluctuations of temperature in an enclosure with a simple on-off control system are discussed. It is shown that such a system can be improved by using two control elements completely immersed in the fluid of the enclosure, and therefore not subject to datum error. The output signal of one of the control elements is required to have a lagging phase angle, and a theoretical analysis is made in which the thermal frequency responses of various configurations are compared. Conclusions are reached about the relative merits of different configurations, and a description is given of the application of the theoretical principles to an experimental temperature-controlled enclosure.