AbstractThe construction is given of an isothermal twin calorimeter in which the small heat absorption on mixing two alkanes is compensated by electric heating. Results are given for the systems n‐hexane‐n‐hexadecane (20° C), n‐heptane‐n‐hexadecane (20°, 25° C), n‐octane‐n‐hexadecane (20°, 30°, 50° C), n‐decane‐n‐hexadecane (20°, 30°, 73° C), n‐decane‐n‐dotriacontane (73° C) and 2,2,4‐trimethyl pentane‐n‐hexadecane (25° C). A recent theory of Tompa s is compared with the observed dependence of the heat of mixing on the difference in chain lengths of the constituent molecules. The strong temperature dependence of the heat of mixing found is discussed in its relation to the entropy of mixing.