Abstract: An apparatus for maintaining a constant temperature within a vessel immersed in a liquid by surrounding a portion of the vessel, extending above the surface of the liquid, with a wick. The wick conducts liquid up to a predetermined point on the vessel and maintains the liquid at that point, regardless of changes in the level of the liquid due to evaporation. The wick is encased within a layer of heat insulating or heat conducting material to further aid in maintaining a constant temperature within the vessel. A second embodiment, including alternating layers of heat conducting and heat insulating material, is disclosed for increased temperature control. The disclosed apparatus is particularly useful for maintaining a constant temperature within a vessel attached to a scientific instrument, such as a pore volume and surface area analyzer.