Composites containing 50% wt fly ash (sourced from the UK and South Africa) in polypropylene homopolymer (manufacturer stabilized for general purpose use) have been prepared by using batch and continuous methods. The effect of the following coupling agents were investigated on the photo- and thermal-decomposition of the composite materials: Lubrizol Solplus C800 (an unsaturated carboxylic acid), γ-methacryloxypropyl trimethoxy silane (γ-MPS), 1,3-phenylene dimaleimide (BMI), and maleic anhydride-grafted-polypropylene (m-PP). High melt, thermal-, and photo-stability was favored when the matrix was coupled to the filler surface by monomeric coupling agents that were expected to adsorb in a close packed layer on the fly ash surface. Further improvements were observed in cases where the coupling agent could also self-polymerize. m-PP did not lead to increased stability due to its low adsorption density on the fly ash surface. The relatively high water/acid soluble transition metal ion content of the UK sourced fly ash did not appear to affect stability under the test conditions employed in this study. The South African sourced fly ash had a higher level of quartz and mullite together with a high level of group 1 and 2 metals. The latter in particular may have led to debonding of the coupled interfacial region from the filler surface and possible adsorption of stabilizers on the pristine surface. This resulted in the South African fly ash generally possessing poorer resistance to oxidation than the UK fly ash. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39974.