Thermal degradation and stabilization of polyethylene glycol (PEG) with a molecular weight of 6000 have been investigated in order to develop PEG as a thermal energy storage material. Low molecular weight eaters including formic esters were produced as the main products of the thermal degradation of PEG at 80 degrees C in air. No degradation was observed for PEG aged in a vacuum. The mechanism of thermal degradation is found to be the random chain scission of the main chain. Thermal degradation could be suppressed by adding 2,2’-methylene-bis(4-methyl-6-tert-butylphenol) (MBMTBP) as an antioxidant. In the stabilizing process, MBMTBP itself is transformed to dimers or trimers.