Aqueous solutions of the triblock copolymers poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) EO13PO30EO13 (Pluronic L64) and EO6PO34EO6 (Pluronic L62) were investigated over a wide polymer concentration range (20-90% w/w) across all phases, from micellar to liquid crystalline and to reverse micellar phases. Electron spin resonance spectra of a homologous series of cationic nitroxide spin probes, CATn (n is the number of carbon atoms in the alkyl substituent), were used to deduce the local polarity in the water-rich domains of the self-assembled polymer aggregates. The isotropic hyperfine splitting from the N-14 nucleus of the >NO fragment, a(N), was the polarity sensitive parameter; a(N) values were translated into effective local hydration values Z(eff) by reference to a series of PEG/water mixtures with water contents expressed as Z [H2O]/[EO]. By this approach it was possible to follow changes in the hydration of the EO blocks at various distances from the hydrophobic core in the L62 and L64 aggregates on a scale of less than or equal to 20 Angstrom and to infer from these changes the mechanism of phase transitions. The results are consistent with the description of the polar part of the system (EO blocks and water) as representing a unique, nonhomogeneous phase with a continuous variation of the hydration level along "estuaries" formed by the hydrated EO blacks. The degree of hydration is higher at 295 K compared to 320 K. While qualitative trends are similar for L62 and L64 phases, all hydration values appear drastically reduced in L62 relative to L64 when compared at the same Z values. However, comparison on a weight percent basis revealed a much closer similarity. This behavior, observed by us in a number of cases and also by other authors, led us to propose that in L62 a number of PO groups are in the polar regions. The detailed local data support the rationalization of the phase diagrams of Pluronics L62 and L64 in terms of changes in the dimensions of the polar head due to hydration and changes in the curvature of the aggregates due to the hydration gradient in the hydrophilic regions.