Abstract: Ethylene oxide-styrene-ethylene oxide triblock polymers (PEO-PSt-PEO)were synthesized via sequential anionic polymerization by using sodium naphthalene as initiator, and the purified PEO-PSt-PEO triblock polymers and their parent polystyrenes were characterized by viscometry, GPC, H-NMR, dn/dc, and solubility measurements. It was found that the intrinsic viscosity and molecular weight of polystyrene inversely increased with the decrease of [M]/[I] ratio in the case of high concentration of initiator, indicating the existence of self-inhibiting action of inifiator. The refractive index increments of PEO-PSt-PEO triblock polymers showed good linear relations with their compositions. Therefore, dn/dc measurements can be used as a reliable method for the composition determination of the block polymers studied. Peak molecular weights of the block polymers in size exclusion chromatograms in THF were very close to that of the corresponding parent polystyrene. PEO-PSt-PEO with lower polystyrene content exhibited poorer solubility in THF.