In aqueous solutions of poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-PEO triblock copolymers (Pluronics P85, F88, and L64), structure formation (micellization) on increasing temperature was followed by determination of rate constants k(iso) of the thermal cis-trans isomerization of 4,4＇-nitroanilinoazobenzene by means of flash photolysis in H2O and D2O. The kinetic solvent isotope effect k(iso,H2O)/k(iso,D2O) indicates that the azobenzene dye molecules are solubilized in a water-rich environment. From the nearly constant solvatochromic UV/vis absorption band maxima lambda(max) of the dye, it is concluded that the S shape of the In k(iso) vs 1/T curves is mainly due to microviscosity changes on micellization. Critical micelle temperature values derived are in satisfactory agreement with those from self-diffusion coefficients of the copolymer molecules dependent on temperature determined by means of pulsed field gradient nuclear magnetic resonance measurements. The self-diffusion experiments allow conclusions on the size of the diffusing particles in H2O and D2O and the influence of dye molecules on aggregation. The hydrodynamic radii of the diffusing species are larger in H2O than in D2O. The reason is seen in the stronger hydrogen bonds between EO units and D2O compared to those between EO units and H2O. On gelation of 25% (w/v) F88 in water at 31 degrees C, the bulk viscosity increases sharply but the microviscosity around the dye molecules does not.