In this work, contamination effects are studied on two polypropylene-based materials. These effects were identified on the molecular weight and on rheological and mechanical properties. Model contaminants chosen in this study were ethylene glycol and engine oil. They were added during the extrusion process to simulate the degradation due to contaminants. To quantify contamination effects and to correlate them with the microstructure evolution, scanning electron microscope (SEM) analyses were also conducted. The analysis of the different obtained results led to a better understanding of the mechanisms involved in the contamination process. Contaminants slightly affect the rheological properties, whereas mechanical properties are more influenced. These observations are confirmed by the modification of the fractured surfaces of the materials due to the presence of contaminants observed on SEM micrographs. POLYM. ENG. SCI., 50:1-9, 2010. (C) 2009 Society of Plastics Engineers