Continuous wave (CW) electron paramagnetic resonance (EPR) studies of the 12-doxylmethyl stearic acid methyl ester (12-DOXYL) spin probe were made in the solutions of a triblock copolymer of acrylamide, vinylchloroethyl ether and an ester of the vinylchloroethyl ether with 2-[(2,6-dichlorophenyl)amino] benzene acetic acid, "dichlophenac" (DPH). Light scattering experiments show that the hydrophobic DPH group influences the clustering of the polymer in solution. The EPR evidence suggests that when the polymer is dissolved in buffer solutions of 12-DOXYL, the spin probe undergoes a partitioning between the solution, the polymer clusters and a minor third environment. The rate of the 12-DOXYL partitioning from the solution into the polymer cluster is fast compared with the time scale of the changes observed in light scattering and exhibits a dependence on the concentration of the polymer in solution and the pH. The 12-DOXYL environment in the polymer cluster is characterized as restricting 12-DOXYL to slow molecular motion with rotational correlation time tau = 1.3 X 10(-8) s/rad and permitting relatively infrequent collisions between the 12-DOXYL molecules. The solution spectrum of the 12-DOXYL is dominated by the spin exchange narrowing expected for concentrated assemblies of the 12-DOXYL molecules, such as a micelle or bilayer. Partitioning of a hydrophobic spin probe in an aqueous solution is shown to be a sensitive measure of the presence of stabilizing hydrophobic regions in a polymer aggregate.