Species loss directly affects the magnitude and stability of various ecosystem processes, and species composition can drive this phenomenon. Much of the evidence that species loss affects ecosystem processes comes from experiments where species richness was manipulated while holding abundance/biomass of individual species constant. Given that species rarely coexist in equal proportions, neglecting evenness might under/overestimate the role of important species combinations. We examined leaf litter breakdown in a small stream based on species-specific input rates of leaf litter from the four dominant species (Liriodendron tulipifera, Fraxinus pennsylvanica, Fagus grandifolia, Quercus prinus), comprising 71% of inputs over peak leaf fall, for a full-factorial litter mixture study. Our experimental approach departed from previous mixture studies in that while we created all two-, three-, and four-species combinations holding species-specific mass constant, we also created a complementary set of mixtures that reflected the natural proportion we estimated from the survey. We found that species richness and evenness alone did not explain variation in breakdown rate, but an interaction between the two did, and mixtures reflecting ambient evenness lost mass nearly 33% faster than single species treatments. Analysis of individual treatments revealed that the emergent effect of mixing species was nearly twice as common in uneven vs. even mixtures. The compositional effects of litter diversity on breakdown uncovered in previous studies might be more pronounced if evenness, and not just richness, is considered when evaluating the role of species loss in these ecosystems.