Ultrasonic relaxation (0.2 < f < 110 MHz) and volumetric studies have been carried out on aqueous solutions of three ethoxylated nonionic surfactants, C(7)E(4), C(8)E(4), and C(8)E(5) (tetra(ethylene glycol) monoheptyl ether and tetra- and penta(ethylene glycol) monooctyl ethers, respectively), as a function of surfactant concentration at 20 degrees C. The ultrasonic relaxation spectra of all systems investigated show two relaxation processes in contrast to those of ionic surfactant solutions, which indicate a single relaxation process. One relaxation process is located in the frequency range 0.16-1.3 MHz and a second in the range 0.3-3.4 MHz. These results disagree with those reported previously in the literature. The amplitudes of the low-frequency relaxation show a peak at concentrations near the cmc and then decrease with increasing the total concentration. The amplitudes of the high-frequency relaxation, on the other hand, are almost independent of the total concentration. Both relaxation frequencies increase linearly with total concentration. The process with the lower relaxation frequency has been assigned to the exchange process of the surfactant monomer between the micelle and bulk solution. The mean dissociation rate constant and magnitude of the volume difference due to the exchange process are determined by applying the relations derived by Aniansson and Wall and by Teubner, respectively. The results of volumetric studies also support this assignment. An unequivocal assignment of the process with high relaxation frequency is difficult at present. Some possible sources of the high-frequency relaxation have been discussed. The most probable source may be an interaction between oxyethylene groups of the surfactant and water that penetrates into the micelle.