plugging of reservoirs with high molecular weight polymers , e . g . partially hydrolyzed polyacrylamide , is overcome by chemically treating a polymer having an excessively high average molecular weight prior to injection into a reservoir with an oxidizing chemical , e . g . sodium hypochlorite , and thereafter incorporating a reducing chemical , e . g ., sodium sulfite , to stop degradation of the polymer when a desired lower average molecular weight and flooding characteristics are attained .