A UK sourced fly ash (coded UKFA) was introduced into polypropylene (PP)/polyethylene (PE) based composites at different PP/PE ratios to investigate its effect on the physical and mechanical properties of the material. Composites containing 65% wt. fly ash modified with commercial peroxide (DCP) and commercial coupling agent (C800) were prepared via batch mixing and compression moulding. The usage of DCP led to the formation of cross-linked PE, which was responsible for highly viscous composites. The newly formed cross-linked PE, together with chain scission of PP, would promote phase separation. Mechanically less stiff materials were produced, especially at high PE levels, because of the larger rubbery interfacial region, despite the good matrix–filler adhesion increasing the strength balance. POLYM. ENG. SCI., 54:1239–1247, 2014. © 2013 Society of Plastics Engineers