Whole hop bines (HBs), the peeled outer bark (OB) of HBs, and fibers chemically extracted from hop bark (HFs) were used as reinforcements to make lightweight composites with polypropylene (PP) webs or fibers as the matrix materials. Using discarded HBs for composites not only increases the value of hop crops but also provides a green, sustainable, and biodegradable material for the composite industry. Lightweight composites are preferred, especially for automotive applications because of the potential energy savings. In this research, the effects of the processing parameters on the properties of PP composites reinforced with HBs were studied. The composites reinforced with OB without any chemical treatment showed better properties than the composites reinforced with HFs or HBs. Compared with jute-PP composites of the same density (0.47 g/cm(3)) composites reinforced with OB had 43% higher flexural strength, 46% higher impact resistance, 50% higher Young's moduli-is, similar modulus of elasticity, 33%. lower tensile strength, and better sound-absorption properties. OB-PP composites with optimized properties have the potential to be used in industrial applications such as support layers in automotive interiors, ceiling tiles, and office panels. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2366-2373, 2010