Thermal and dynamic mechanical behaviors of wood plastic composites made of poly vinyl chloride (PVC) and surface treated, untreated wood flour were characterized by using differential scanning calorimetry and dynamic mechanical analysis. Glass transition temperature (T-g) of PVC was slightly increased by the addition of wood flour and by wood flour surface treatments. Heat capacity differences (Delta C-p) of composites before and after glass transition were markedly reduced. PVC/wood composites exhibited smaller tan 6 peaks than PVC alone, suggesting that less energy was dissipated for coordinated movements and disentanglements of PVC polymer chains in the composites. The rubbery plateaus of storage modulus (E') curves almost disappeared for PVC/wood composites in contrast to a well defined plateau range for pure PVC. It is proposed that wood flour particles act as "physical crosslinking points" or,pinning centers" inside the PVC matrix, resulting in the absence of the rubbery plateau and high E' above T-g The mobility of PVC chain segments were further retarded by the presence of surface modified wood flour. (C) 2007 Wiley Periodicals, Inc.