The formation and structure of micelles by an amphiphilic siloxane-graft-polyether copolymer in water mixed with a polar organic solvent were investigated by a variety of experimental techniques. The cosolvents examined, of interest to waterborne ink and coating formulations, were ethanol, 2-propanol, glycerol, ethylene glycol, propylene glycol, formamide, sulfolane, and 2-pyrrolidone, and their concentration in water ranged up to 20%. We measured the cmc (critical micellization concentration) by three different methods: diphenylhexatriene solubilization, methyl orange hypsochromic shift, and pyrene fluorescence 11113 ratio. These methods gave the same trends on cmc in various cosolvents. The added cosolvents generally cause an increase in the cmc of the siloxane copolymer. Cosolvent physicochemical characteristics such as octanol-water partition coefficient and solubility parameter were used to correlate the cosolvent effects. The component of the solubility parameter that accounts for hydrogen bonding is found to best describe the cosolvent effects on the cmc. The microviscosity of the micelles was evaluated by 1,6-diphenyl-1,3,5-hexatriene anisotropy values that were obtained by fluorescence polarization measurements. A lowering in the microviscosity was observed in the presence of cosolvents such as ethanol or 2-propanol. The micelle hydrodynamic radius and micelle size distribution were monitored by dynamic light scattering (DLS). The addition of ethanol resulted in significant reduction in the micelle size, but glycerol or formamide did not have strong effects on the micelle size. The micelle association numbers were estimated from the hydrodynamic radius obtained from DLS.