Optimization of micellar molecular encapsulation systems, such as drug delivery vehicles, can be achieved through fundamental understanding of block copolymer micelle structure and dynamics. Herein, we present a study of PEO PCL block copolymer spherical micelles that self-assemble at 1% wt/vol in D2O THE-d(8) mixtures. Varying solvent composition as a function of cosolvent THE- d(8) at constant polymer concentration (1% wt/vol) allows sensitive study of how small molecule additives influence micelle structure and dynamics. We conduct nuclear magnetic resonance spectroscopy and diffusometry on two block copolymer (2k series: PEO2k-PCL3k; 5k series: PEO5k-PCL8k) spherical micelles that show drastically different behaviors. Unimers and micelles coexist in solution from 10-60 vol % THE-d(8) for the 2k series but only coexist at 60 vol % THE-d(8) for the 5k series. At >= 60 vol % THE-d, micelles disassemble into free unimers for both series. We observe relatively flat micelle diffusion coefficients (similar to 1 x 10(-10) m(2)/s) with increasing THE-d(8) below 60 vol % for both 2k and 5k series, with only small changes in micelle hydrodynamic radius (P-(approximate to 14 nm) over this range. We compare these results to a detailed SANS and microscopy study described in a companion paper. These fundamental molecular dynamics, unimer population, and diffusion results, as a function of polymer composition and solution environment, provide critical fodder for controlled design of block copolymer self-assembly.