Poly(siloxyethylene glycol) (PSEG), which consists of alternating oligo(dimethylsiloxane) and oligo(ethylene glycol) (OEG) units in the main chain, was synthesized through polycondensation reactions between oligosiloxane having two diethylamino groups and OEG. The molecular weight of the polymer was in the range 3500-17 700 g/mol with a monomodal molecular weight distribution. The glass transition temperature of the polymer thus obtained was controlled by changing the silicon content in the polymer. Actually, with increasing Si content, the T-g of the polymer decreased from -53 (PEG) to -100 degrees C. PSEG having a silicon content of <15 wt % was soluble in aqueous media. With increasing solution temperature, phase separation occurred and the solution become turbid. The temperature at the phase separation is known as a lower critical solution temperature (LCST). The LCST was controlled by the silicon content in the polymer. The hydrolytic stabilities of the PSEG in aqueous solution were also examined.