The structure and the rheology of polymeric micelles in aqueous solution were studied using dynamic and static light scattering, small-angle X-ray scattering, and oscillatory shear rheology. The polymeric micelles were formed by association of poly(ethylene oxide) end-capped with octadecyl. These systems show a discontinuous liquid-solid transition at high concentrations and low temperatures. In the solid state domains appear where the micelles have a crystalline order. Two different molar masses of PEO were used, and mixtures with various ratios were investigated. It is shown that in all cases mixed micelles are formed, which influences both the liquid-solid transition and crystal structure. Incorporating small chains in micelles formed by large chains or vice versa allows in both cases to the micelles to flow and leads to disappearance of the crystalline order, but through different mechanisms.