A novel reaction mechanism of H 2 and CH 4 cogeneration from water hyacinth (Eichhornia crassipes) was originally proposed to increase the energy conversion efficiency. The glucose and xylose hydrolysates derived from cellulose and hemicellulose are fermented to cogenerate H 2 and CH 4 by two-step anaerobic fermentation. The total volatile solid of hyacinth leaves can theoretically cogenerate H 2 and CH 4 yields of 303 ml-H 2 /g-TVS and 211 ml-CH 4 / g-TVS, which dramatically increases the theoretical energy conversion efficiency from 19.1% in only H 2 production to 63.1%. When hyacinth leaves are pretreated with 3 wt% NaOH and cellulase in experiments, the cogeneration of H 2 (51.7 ml-H 2 /g-TVS) and CH 4 (143.4 ml-CH 4 / g-TVS) markedly increases the energy conversion efficiency from 3.3% in only H 2 production to 33.2%. Hyacinth leaves, which have the most cellulose and hemicellulose and the least lignin and ash, give the highest H 2 and CH 4 yields, while hyacinth roots, which have the most ash and the least cellulose and hemicellulose, give the lowest H 2 and CH 4 yields.