One of the materials most commonly found in municipal plastic waste (MPW) is High Density Polyethylene (HDPE), which is used particularly for food packaging and cleaning product containers. Postconsumer HDPE is generally contaminated with other impurities and, therefore, a cleaning process is necessary before it can be reprocessed. The aim of this study was to investigate the influence of the cleaning conditions on the oxidative degradation of postconsumer HDPE. These plastic materials were washed in NaOH solution, liquid detergent (D), and a mixture of NaOH/D (1 : 1). Results for the melt flow rate (MFR) and capillary rheometry showed that NaOH has a great influence on the oxidative degradation of HDPE. The HDPE treated with NaOH solution had a lower decomposition temperature, observed through Thermal Gravimetric (TG) analysis, and a lower the Oxidative Induction Time (OIT), obtained via Differential Scanning Calorimeter (DSC). Finally, the oxidative degradation of HDPE was verified through a higher carbonyl index. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 1454-1460, 2009