Substances containing chlorine are used at low concentrations in fresh water to avoid the spreading of infections [1]. The benefit is that many human lives are saved. The negative consequence is that essentially all materials in contact with the chlorinated medium deteriorate relatively rapidly. Many strongly oxidative species can be present in chlorinated water and polymers in contact with such media may deteriorate by oxidation with an accompanying loss of fracture toughness. This paper presents data for a modern medium- density polyethylene stabilized with a combination of hindered phenolic and phosphite antioxidants. The rate at which the stabi- lizing system is depleted in two different chlorinated media, Cl2 in water and ClO2 in water, is the first topic. The second topic concerns the progression of polymer degradation, where a comparison is made between the degradation scenarios of the polymer in these two media. The third topic is crack propagation and how it is assisted by polymer degradation.