Fermentation of acid hydrolysate of water-hyacinth (Eichhornia crassipes), a free floating aquatic plant has been investigated for ethanol production. The dilute acid treatment has been applied to utilize the maximum hemicellulosic content of the water-hyacinth. The goal of this work was to investigate, both experimentally and theoretically using mathematical tools, a fermentative system utilizing water-hyacinth (Eichhornia crassipes) hemicellulose acid hydrolysate as a substrate for ethanol production using Pichia stipitis. It was found that 72.83% of xylose was converted to ethanol with a yield of 0.425 g(p)/g(s) and productivity of 0.176 g(p)/L/h. An appropriate mathematical model was developed to explain theoretically the bioconversion of this hemicellulose acid hydrolysate to ethanol and the model was tested statistically to check the validity of the model. (C) 2009 Elsevier Ltd. All rights reserved.