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 D 2 O−THF-d 8 mixtures. Varying solvent composition as a function of cosolvent THF-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: PEO 2k −PCL 3k ; 5k series: PEO 5k −PCL 8k ) spherical micelles that show drastically different behaviors. Unimers and micelles coexist in solution from 10−60 vol % THF-d 8 for the 2k series but only coexist at 60 vol % THF-d 8 for the 5k series. At ≥ 60 vol % THF-d 8 micelles disassemble into free unimers for both series. We observe relatively flat micelle diffusion coefficients (∼1 × 10 −10 m 2 /s) with increasing THF-d 8 below 60 vol % for both 2k and 5k series, with only small changes in micelle hydrodynamic radius (≈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.