Thermal changes in the association behavior of poly(ethylene oxide) poly(propylene oxide) alternating multi block (PEO PPO AMB) copolymers in water are investigated by the use of transmittance and light scattering measurements. Two PEO PPO AMB copolymers with a different weight fraction of PEO, (EO220PO33)(8) and (EO68PO33)(9), are prepared. The weight-average molecular weights of (EO220PO33)(8) and (EO68PO33)(9) estimated by static light scattering measurements are 1.3 X 10(5) and 4.1 X 10(4) g molt, respectively. The number of PEO PPO repeating pairs is over 8. It is found that the aqueous solution of (EO220PO33)(8) undergoes phase separation with a lower critical solution temperature (LCST) of around 58 degrees C at 0.3 LCST is estimated to be ca. 42 degrees C. The critical solution concentration wt %. For the aqueous (EO68PO33)(9) solution, the for (EO68PO33)(9) is not clear because of a small concentration dependence of TT at a higher concentration range. Dynamic light scattering measurements indicate that a micellelike aggregate is formed below the LCST. From the Debye plot, it is elucidated that the second virial coefficient, A2, starts going down at around 32 degrees C for (EO220PO33)(8) and below 15 degrees C for (EO68PO33)(9). The A2 value of (E0220P033)8 approaches 0 near 50 degrees C, whereas that of (EO68PO33)(9) approaches 0 at around 35 degrees C. At a high temperature, the attractive interaction among the copolymers becomes dominant, thereby inducing the formation of micellelike aggregates.