Phase behavior is established for a block copolymer polyethyleneoxide-b-dimethylsiloxane-polyethylenoxide (EO)(15)-(PDMS)(15)-(EO)(15) (IM-22) a in glycerol/water mixed solvent. In water alone, the block copolymer forms biphasic micellar/lamellar (L-1/L-alpha) systems over the range 10-70 wt%, with single L-alpha-phases between 70-90 wt%. Strong solvent effects on the phase behavior were noted. For example, using a mixed 60:40 vol% glycerol/water solvent, the single L-alpha-phase region appears at Much lower concentrations, only 20 wt% IM-22, as compared to the biphasic L-1/L-alpha system observed in water alone. This interesting observation of L-alpha-phase swelling on addition of glycerol may be explained by a decrease in attraction between the bilayers, as it is also found that in this mixed glycerol/water solvent there is a close refractive index matching with IM-22. Rheological measurements show the L-alpha-phases with added glycerol have low shear moduli. The influence of added ionic surfactant sodium dodecylsulfate (SDS) on these swollen IM-22 L-alpha-phases was studied. Small-angle X-ray scattering (SAXS) indicated the interlamellar distance d remains essentially constant up to 3 mM SDS, and then decreases with increasing SDS content. This weak effect is consistent with the fact that the L-alpha-phases are most swollen when the mixed solvent contains 60 vol% glycerol. The results suggest that glycerol/water solvent mixtures can be used to tune the refractive index of the background solvent, modifying DLVO-type interactions, and causing significant effects on the phase stability of simple block-copolymer systems. (C) 2007 Elsevier Inc. All rights reserved.