This paper describes the design of the first calorimeter capable of measuring heats of adsorption of gases on single‐crystal surfaces in ultrahigh vacuum. The calorimeter is of the quasi‐adiabatic type, and measures the temperature rise as gas adsorbs. The calorimeter has been used to measure the heat of adsorption of CO and C2H4on Pt(111). The calorimeter seemed to perform quite well in these experiments, and yielded results that are reasonable. Thus, it is suggested that a calorimeter would be a useful addition to a surface scientist’s arsenal.