SummaryThe isotope partition coefficient a between cyclohexane, cycloheptane and hydrogen has been determined between 80 and 180" and pressures around 1 atm. The values decrease with increasing deuterium content y of the hydrogen and can be expressed by the simple equation Inn = A S / R + ( A H +~H . Y)/RTwith an error of 1%. However it was not possible to achieve good agreement with theoretical calculations using a unique average value for the partition function of the hydrocarbon.Introduction. -In an earlier communication we reported on the isotope partition coefficient a in methylcyclohexane/hydrogen mixtures over a wide range of temperature, pressure and deuterium atom fraction of hydrogen (y) and methylcyclohexane (x) [2]. It is defined by the following equation: