A one-way analysis of variance and thermal analysis were performed in this study to examine the influences of the contents, types (exothermic vs. endothermic), and forms (pure vs. masterbatch) of chemical foaming agents (CFAs), as well as the use of coupling agents, on the density reduction (or void fraction) and cell morphology of extrusion-foamed neat high density polyethylene (HDPE) and HDPE/wood-flour composites. The CFA types and forms did not affect the void fractions of both the neat HDPE and HDPE/wood-flour composites. However, a gas containment limit was observed for neat HDPE foams whereas the average cell size achieved in the HDPE/wood-flour composite foams remained insensitive to the CFA contents, irrespective of the foaming agent types. The experimental results indicated that the use of coupling agent in the formulation was required to achieve HDPE/wood-flour composite foams with high void fraction. (C) 2003 Wiley Periodicals, Inc.