Polypropylene (PP)-based wood polymer composites with 40 and 60 m % wood content, respectively, have been prepared, employing two different wood types and 0.0-20.0% (based on wood content) of a maleated polypropylene (MA-PP) coupling agent (CA). The main objective of this research was to find out if the optimum CA content is dependent on wood filler particle size. Tensile and flexural properties, as well as impact strength, heat deflection temperature, and water absorption served as indicator properties for the CA influence. For all properties, there is an optimum or the onset of a plateau occurring between 8.0 and 13.0 m % MA-PP content. Flexural properties were determined after 28 days of water submersion as well, for both wet and redried specimens, revealing that the mechanical properties can be restored to a large extent in sufficiently coupled compounds. A dependence of the MA-PP content optimum on particle size was not detected. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013