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Litter mixtures often decompose at a different rate than the average of the individual litters, but, so far, the underlying mechanisms are not understood. We propose here an explanation based on a model with two litters. The model describes the carbon and nitrogen mineralisation of the litters. The decomposition rates of the litters become linked because the growth efficiency (production-to-assimilation ratio) of the decomposers responds to the amount of inorganic nitrogen (initial plus mineralised) in the surrounding environment. The model shows that, when in a mixture, one litter decomposes always faster and the other one always slower compared to when they decompose on their own. The relative changes in decomposition rates are also equal and consequently the decomposition rate for the whole mixture can be expected to lie between the rates of the two individual litters. The mixture decomposes faster than the average of the two litters separately when the litter of the higher quality also mineralises nitrogen fastest. If the litter of the higher quality instead has the smallest nitrogen mineralisation rate, the mixture decomposes slower. The model predictions are consistent with observations from 23 published experimental litter-mixture studies.