In natural ecosystems plant litter is typically a mixture of more than one species and the rate of decomposition can be faster (synergistic) or slower (antagonistic) than the average of its component species (non-additive effects). We analysed the decomposition rates of two-species mixtures to determine if there were consistent non-additive effects of litter mixing on decomposition and how do they compare with the effects of species identity on mixture decomposition. Then we tested if non-additive effects were consistently associated with the presence of particular species in the mixture, to the combination of Fast- or Slow-decomposing species, or to initial litter quality of mixtures. We found: (a) that species identity was the primary determinant of the decomposition rate of mixtures, and (b) we detected significant, but weak, non-additive effects which were consistently synergistic in the most chemically heterogeneous mixtures. However, slower decomposing species appeared to increase the decomposition rate of faster decomposing species (30 times out of 41 after 2 months of incubation, and 17 times out of 24 after 9 months of incubation). During the initial stages of decomposition, low-lignin mixtures showed mostly synergistic effects, whereas high-lignin mixtures showed antagonistic effects. At more advanced stages of decomposition, mixtures containing species with highest difference in initial N content had more synergistic effects, whereas those with similar initial N content showed both synergistic and antagonistic effects. Our results confirm previous findings about the importance of chemical heterogeneity of mixtures as a driver of decomposition rates of litter mixtures. We propose that mechanisms related to carbon priming may be related to synergistic effects in most heterogeneous mixtures, while nitrogen interaction with carbon may be resulting in antagonistic effects in homogeneous and Slow-decomposing species mixtures.