The importance of leaf litter diversity for decomposition, an important process in terrestrial ecosystems, is much debated. Previous leaf litter-mixing studies have shown that non-additive leaf litter diversity effects can occur, but it is not clear why they occurred in only half of the studies and which underlying mechanisms can explain these conflicting results. We hypothesized that incorporating the role of macro-detritivores could be important. Although often ignored, macro-detritivores are known to strongly influence decomposition. To better understand the importance of macro-detritivores for leaf litter mixing effects during decomposition, four common leaf litter species were added separately and in two and four species combinations to monocultures of three different macro-detritivores and a control without fauna. Our results clearly show that leaf litter-mixing effects occurred only in the presence of two macro-detritivores (earthworms and woodlice). Application of the additive partitioning method revealed that in the specific combination of woodlice and the presence of a slow-decomposing leaf litter species in the mixture, leaf litter mixing effects were strongly driven by a selection effect. This was caused by food preference of the isopod: the animals avoided the slow decomposing species when given the choice. However, most leaf litter mixing effects were caused by complementarity effects. The potential mechanisms underlying the complementarity effects are discussed. Our results clearly show that that both leaf litter and macro-detritivore identity can affect litter diversity. This may help to explain the conflicting results obtained in previous experiments.