By means of microsecond flash photolysis the rate constants, k iso, of the thermal cis → trans isomerization of two azo dyes were determined in dependence on temperature in aqueous solutions of poly(ethylene glycols) (PEGs) and of a poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) block copolymer (F88, EO96PO39EO96). PEG/water mixtures are considered as a model for the PEO/water interface layer of micelles of F88. The dependence of k iso on bulk viscosity of PEG/water mixtures varying with the number of water molecules, Z, per ethylene oxide unit is stronger than that in water-free solutions, and the kinetic solvent isotope effect (k iso,H 2 O/k iso,D 2 O) is larger than the viscosity solvent isotope effect of the mixtures (η D 2 O/η H 2 O). Therefore we assume that beside changes of microviscosity a dynamic hydrogen bonding effect of water contributes to the sensitivity of k iso on nanostructure changes in aqueous surfactant solutions. Comparing temperature-dependent k iso, Z data from PEG/water mixtures with k iso values determined in aqueous solutions of F88, hydration numbers Z in the microenvironment of isomerizing dyes were approximately estimated. From S-shaped Arrhenius plots of k iso critical micellization temperatures were determined, and the dependence of k iso data on copolymer concentration is discussed.