Solutions of poly(2-ethyl-2-oxazoline-dimethylsiloxane), PEOX-PDMS, diblock copolymers in water-2-propanol mixtures were studied by dynamic light scattering. A diblock consisting of PDMS block with M(n) = 4.8 kg mol(-1) and a PEOX/PDMS degree of polymerization ratio 3 displayed three regimes in mixtures of water and 2-propanol. Compact micelles formed in pure water and at low mole fractions of 2-propanol where the hydrophobic PDMS blocks formed the core and hydrophilic PEOX blocks formed the corona. At a critical 2-propanol mole fraction and at C-p greater than or equal to 1 g L(-1), anomalously large micelles were observed. At higher a-propanol concentrations, the micelles dissociated to form either single or slightly associated chains due to increasing solvation of the PDMS blocks. A second diblock consisting of a PDMS block with M(n) = 2.2 kg mol(-1) and a PEOX/PDMS degree of polymerization ratio = 3.7 formed structures with unusually large hydrodynamic diameters in pure water and in mixtures of water and 2-propanol.