The saccharification of microcrystalline cellulose by reconstituted ternary mixtures of purified cellulases (one endoglucanase and two cellobiohydrolases) has been studied over the entire range of mixture compositions. Ternary plots are used to compare the performance of five synthetic mixtures drawn from the cellulase systems of Acidothermus cellulolyticus, Trichoderma reesei, Thermomonospora fusca, and Thermotoga neapolitana. Results reveal that at least one synthetic mixture utilizing enzymes from three different organisms delivers performance competitive with that of a "native" (i.e., co-evolved) ternary system drawn exclusively from T. reesei. This heterologous system, consisting of the endoglucanase E1 from A. cellulolyticus and the exoglucanases CBHI from T. reesei and E3 from T. fusca, is forgiving from the system-design point of view, in that it delivers high saccharification rates over a wide range of mixture compositions.