The activation energies for the exchange reactions between sodium iodide and cyclopentyl, cycloheptyl, cycloöctyl, cyclopentadecyl, and 2-heptyl iodides were all in the range 21 ± 1 kcal. per mole. For cyclohexyl iodide the value was 25.6 kcal. per mole. This difference seems to be explained from a consideration of the initial potential energies of the various rings. An inverse relation appears to exist between log PZ and [Formula: see text], the reciprocal of the number of carbon atoms in the iodides investigated, for all the iodides except cyclopentyl iodide. This might be due to a change in the number of equivalent constellations in the activated state.