A mixed tree species composition is frequently proposed as a way to increase habitat heterogeneity and support greater biodiversity in commercial forests. However, although international forest policy is increasingly advocating stands of mixed tree species, there is evidence to question the biodiversity benefits conferred by such forests. Using active ground-dwelling spiders and carabid beetles as biodiversity indicator taxa, we investigated the effect of forest stand composition on spider and carabid beetle community structure and composition. We conducted pitfall trapping in the summer of 2011 in 42 plantation forest stands across three different geographical regions in the UK and Ireland. Three common plantation forest stand types were examined: oak monocultures, Scots pine monocultures, and intimate Scots pine and oak mixtures (oak ⩽60% cover). Forest stand type had a weak effect on spider and beetle species richness, with no significant differences in mixed stands compared with monocultures. There were few differences in species composition between the stand types in each region and indicator species analysis found few species specifically affiliated with any of the forest stand types. Land use history is hypothesised to have contributed, at least in part, to the observed important regional differences in spider and beetle assemblages. Our results do not support the perception that intimate mixtures of dominant tree species benefit biodiversity in plantation forest stands. Further research is required to determine the optimum percentages and planting patterns required for mixtures of canopy tree species in order to support forest biodiversity.