We study the phase behavior and microstructure of alkyl-β- monoglucosides with intermediate chain lengths (octyland nonyl-β-glucoside) in aqueous solutions containing ammonium sulfate and poly(ethylene glycol) (PEG). When the glucoside surfactants are mixed with PEG of molecular weight 3350 or larger, two different phase transitions are observed in the temperature range 0-100° C, with lower and upper miscibility gaps separated by a one-phase isotropic region. Isothermal titration calorimetry is used to quantify the effect of PEG on the micellization properties of the alkyl monoglucosides, whereas small-angle neutron scattering gives insight into the microstructure of the surfactant/ polymer mixtures near the liquid-liquid phase boundary. Results show that the range and the strength of the interactions in these solutions are highly affected by the presence of PEG, Solutions with nonyl-β-glucoside contain larger micelles than those with octyl-β-glucoside, and the intermicellar interactions are much stronger and longer ranged. The relevance of these findings for membrane protein crystallization is discussed.