A comprehensive study of sound velocity and apparent specific volume, and adiabatic compressibility and expansibility thermodynamic properties of aqueous solutions of a number of POE-POP-POE triblock copolymers has been carried out in the temperature range 10-45 degrees C. Three series of the above block copolymers were studied : one having similar relative molar masses but different POP/POE mass ratios (F38, P103, P85); another having the same POP/POE mass ratio but different relative molar masses (L64, P84, P104 and F38, F68, F108); and a series in which the copolymers have the same absolute POP content but different relative molar masses (P103, P104, P108). An abrupt transition in the temperature dependence of the volumetric and sound velocity properties occurs in the temperature range 10-45 degrees C for many of the copolymers. This is due to the self-aggregation of the copolymers. The results of this study show that the relative molar mass, POP/POE mass ratio and POP content of the copolymers are factors that play a role in the self-aggregation of these systems. The extensive study of the sound velocity property of these systems extends and confirms the few previous reports that it is a sensitive macroscopic probe of the onset of self-aggregation processes in these systems. The apparent specific volume data of the copolymers has been used to test current models of these aggregates. The results show that the POP core contains some water.