Mutual diffusion coefficients (D) and self-diffusion coefficients (D') in D,Osolutions of n-dodecylpolyoxyethylene surfactants [C,,H,,(OC,H,),OH, m = 5 and 81 have been measured by dynamic light scattering and pulsedgradient F.t.n.m.r., respectively. On the basis of the ratio DID', micelle size distribution and intermicellar interactions have been discussed. The results have been compared with those for the C12E, system reported previously. Below ca. T,-30 "C (T, is the lower critical solution temperature) size distribution is narrow and intermicellar interactions are weak; this is consistent with the existence of spherical micelles. In the temperature range T, -30-T, -15 "C, the excluded volume effects are dominant and increase in magnitude with increasing temperature. At the same time, the size distribution becomes broad. These results originate from elongation of micelles. Above ca. T,-15 "C, the results can be explained by the decrease in the excluded volume effects and/or rapid increase in attractive interactions with increasing temperature. At still higher temperatures (T, -4 "C-T,), attractive interactions become dominant. Proton n.m.r. linewidths and viscosities have also been measured.