Many studies testing hypotheses about the effects of leaf litter diversity on litter processing rates contain a systematic bias that stems from mathematically incorrect assumptions. Assuming that the mean of single-species processing rates adequately describes noninteracting mixed-species rates will consistently underestimate or miss positive interactions between species. Further, the slope of log-transformed mass loss of litter mixtures vs time is affected by the duration of the experiment. Simple solutions to these errors include separating species from litter mixtures after incubation, or comparing masses remaining rather than comparing slopes.