The dynamics of the micelles of five triblock poly(ethyleneoxide)poly(propyleneoxide)-poly(ethyleneoxide) copolymers, the Pluronics P104 (EO27PO61EO27), P84 (EO19PO43EO19), P65 (EO18PO29EO18), P85 (EO26PO40EO26), and P103 (EO17PO60EO17), have been investigated using two chemical relaxation methods: the temperature-jump and the ultrasonic relaxation (absorption). In the frequency range investigated (0.5-50 MHz), the ultrasonic absorption spectra (absorption vs frequency plots) consisted in tails of relaxation curves, indicating characteristic times much longer than 0.3 mu s for the exchange of copolymers between micelles and intermicellar solution. Absorption measurements at a fixed frequency yielded the critical micellization temperature of the solutions. The temperature-jump results obtained in this study together with those from a previous one for the copolymers L64 (EO13PO30EO13) and PF80 (EO73PO27EO73) (B. Michels et al., Langmuir 13, 3111, 1997) showed that the relaxation time associated with the formation/breakup of micelles becomes longer upon increasing copolymer molecular weight at constant composition. This time also increased when decreasing the length of the hydrophilic block at fixed hydrophobic block length or increasing the length of the hydrophobic block at fixed hydrophilic block length, similar to conventional surfactants. The dynamics of block copolymers micelles in aqueous solution are discussed.