This study investigated the direct supplementation of sodium chlorite (NaClPO2) in acidic fermentation process rather than using NaClO2 as a pretreatment agent. This approach targets saving both of the cost of external acetic acid that needs to be added in pretreatment and the loss of polysaccharide, which simultaneously occurs during pretreatment process. Whereas, fermentation of water hyacinth (WH) provided quite low H-2 yield (HY) of 39.7 +/- 2.5 mL/g(TVS) and lignin destruction of 2.6 +/- 0.2%. Cellulase and xylanase enzymes amounted to 2.15 +/- 0.18 and 1.81 +/- 0.14 U/mL, respectively. Enterobacter and Clostridium sp. accounted for 2.41 +/- 0.15 x 10(5) and 4.02 +/- 0.27 x 10(4) cfu/mL, respectively. However, NaClO2 addition significantly augmented HY, cellulase and xylanase enzymes to 119.6 +/- 7.8 mL/g(TVS), 3.46 +/- 0.21 and 2.09 +/- 0.23 U/mL at dosage of 8.0 mg/L, respectively. Moreover, cellulose, hemicellulose and lignin degradation efficiencies were maximized to 57.8 +/- 3.1, 46.3 +/- 3.8 and 31.6 +/- 1.6%, respectively. Enterobacter and Clostridium sp. counts were 4.23 +/- 0.28 x 10(5) and 9.75 +/- 0.52 x 10(4) cfu/mL, respectively in the batches supplemented with NaClO2. Nevertheless, at a dosage exceeding 8.0 mg/L, the HY (26.1 +/- 2.0 mL/g(TVS)) and bacterial count was highly deteriorated due to the excessive production of inhibitory phenolic compounds of 1.61 +/- 0.12 g/L in the medium where the destruction of lignin was quite high (38.8 +/- 2.4%).