Abstract: A thermostat (10) comprising a metal hydride (26) encased in a bellows (16) and a second bellows (20) encasing available hydrogen gas (32) to be adsorped and desorped by the metal hydride (26). The metal hydride (26) mechanically expands its encasing bellows (16) during adsorption and contracts during desorption of the hydrogen gas while a change in gas pressure due to this adsorption/desorption expands and contracts its encasing bellows (20). Each bellows (16,20) is oppositely acting and each engage a valve seat (36,40), respectively, so that the flow of fluid is directed alternately through one valve seat opening or the other depending upon temperature affecting the gas absorping/desorping properties of the metallic hydride. The temperature at which the thermostat (10) operates may be adjusted by controlling the pressure of the gas in the gas bellows. Also disclosed is a variation of the invention as a thermal switch for space applications.