The aim of this research is to propose an improved method to partition single species contributions to decomposition in mixed species litters and to detect additive or non-additive responses in litter decomposition. Using simulated data, we demonstrate that additive responses can arise from multiple conditions, including no changes in litter decomposition rates of both species in the mixtures, or an enhanced decomposition of one species and a reduced decomposition of another. Antagonistic responses can be caused by reduced decomposition of only one species, or of both species. Without partitioning of the contributions of single species proposed here, it is difficult to distinguish the different causes of the overall responses. Our analyses provide a better understanding of litter decomposition in mixtures and have significant implications for modeling litter decomposition.