ABSTRACT Due to rapid depletion of fossil fuels and fluctuating market prices of crude oil, extensive research is going on worldwide to find out alternative renewable fuels that can either completely replace the fossil fuels or that can be blended in certain proportions with the fossil fuels without having major modifications in the engines. The most popular alternative liquid fuels are biodiesel and ethanol. However, both of these have limitations that they can be blended with petro-diesel and gasoline only up to 20%. They also suffer from other limitations such as separation from petrol at low temperature and low heat content that reduces economy of blended fuel. A new alternative fuel that has emerged in recent past is biobutanol, which overcomes the problems faced with biodiesel and bioethanol. Biobutanol is manufactured through the process of ABE (acetone-butanol-ethanol) fermentation using various substrates. In this review, we have compared various processes and the substrates used by them from viewpoint of unit price of the butanol. This analysis is based on published literature, but still gives a view into the niche areas for improving economy of the ABE fermentation process and the biobutanol fuel
