The critical micelle concentrations (cmc’s) of two gemini surfactants, the propane- and hexanediyl-alpha,omega-bis(octyldimethylammonium bromide), referred to as 8-3-8 and 8-6-8, have been determined by spectrofluorometry, ultrasonic absorption, and electrical conductivity measurements. The isothermal volume change upon micellization of 8-6-8 has been obtained from density measurements. The kinetics of the exchange of 8- 3-8 and 8-6-8 between micelles and bulk phase has been investigated by the ultrasonic absorption relaxation method in the concentration range from slightly below cmc to about 2 cmc. All the solutions investigated showed that in the frequency range 0.5-85 MHz a single relaxation process is associated with the exchange equilibrium as in the case of conventional surfactants. It therefore appears that the two alkyl chains of gemini surfactants exit from and enter into the micelles simultaneously. The rate constants for these processes are close to those found for conventional cationic surfactants of similar nature and having nearly the same cmc as the gemini surfactants.