Molecular weight dependence of phase separation behavior of the Poly (ethylene oxide) (PEO)/Poly(ethylene oxide-block-dimethylsiloxane) (P(EO-b-DMS)) blends was investigated by both experimental and theoretical methods. The cloud point curves of PEO/P(EO-b-DMS) blends were obtained by turbidity method. Based on Sanchez-Lacombe lattice fluid theory (SLLFT), the adjustable parameter, epsilon*(12)/k (quantifying the interaction energy between different components), was evaluated by fitting the experimental data in phase diagrams. To calculate the spinodals, binodals, and the volume changes of mixing for these blends, three modified combining rules of the scaling parameters for the block copolymer were introduced. The calculated binodals with those modified combining rules show good agreement with the experimental data. Furthermore, the calculated volume changes during mixing decrease with increasing molecular weight of PEO, and the relationship between the volume changes and temperature is quite different for the mixtures with different molecular weight of PEO. (C) 2008 Wiley Periodicals, Inc.