The use of hydrogen (H<
 sub>
 2<
 /sub>
 ) as a fuel offers enhanced energy conversion efficiency and tremendous potential to decrease greenhouse gas emissions, but producing it in a distributed, carbon-neutral, low-cost manner requires new technologies. Herein we demonstrate the complete conversion of glucose and xylose from plant biomass to H<
 sub>
 2<
 /sub>
  and CO<
 sub>
 2<
 /sub>
  based on an in vitro synthetic enzymatic pathway. Glucose and xylose were simultaneously converted to H<
 sub>
 2<
 /sub>
  with a yield of two H<
 sub>
 2<
 /sub>
  per carbon, the maximum possible yield. Parameters of a nonlinear kinetic model were fitted with experimental data using a genetic algorithm, and a global sensitivity analysis was used to identify the enzymes that have the greatest impact on reaction rate and yield. After optimizing enzyme loadings using this model, volumetric H<
 sub>
 2<
 /sub>
  productivity was increased 3-fold to 32 mmol H<
 sub>
 2<
 /sub>
 ?L<
 sup>
 ?1<
 /sup>
 ?h<
 sup>
 ?1<
 /sup>
 . The productivity was further enhanced to 54 mmol H<
 sub>
 2<
 /sub>
 ?L<
 sup>
 ?1<
 /sup>
 ?h<
 sup>
 ?1<
 /sup>
  by increasing reaction temperature, substrate, and enzyme concentrations?an increase of 67-fold compared with the initial studies using this method. The production of hydrogen from locally produced biomass is a promising means to achieve global green energy production.