The geometrical and structural isomerizations of cyclopropane and the decomposition of cyclobutane are considered from the standpoint of the RRKM theory. The fall-off curves depend critically on the high-pressure Arrhenius parameters; the use of the latest values leads to good agreement with experiment in most cases. Whether or not a biradical mechanism is assumed, there is little effect on the calculated fall-off curves. The treatments are applied to isotope effects as functions of pressure and temperature. The activation energy difference ED–EH varies with pressure in a complicated way, a conclusion that throws light on apparent discrepancies between the results of different workers.