We synthesized water-soluble polyperoxides (PP) by radical copolymerization of hydrophilic sorbic esters with oxygen. A phase separation with a lower critical Solution temperature (LCST) was observed under the onset temperature of thermal degradation for the aqueous solutions of PP containing a tetra(ethylene oxide) unit in the side chain. A critical temperature for the LCST-type phase separation depended on the composition of the hydrophilic and hydrophobic esters of PP.