Due to the continual rise in the cost of crude oil as main energy source, research into sustainable economical and environmental alternatives to fossil fuels is continuously becoming more and more important. One set of promising alternatives to petroleum derived fuels are biofuels,especially those produced via fermentation processes from renewable resources, such as butanol (biobutanol). Biofuels are generally considered as fuel additives rather than petroleum substitutes. The purpose of the present study is to provide further insights on the particular effects of butyric acid added as co-metabolic substrate in the fermentation medium in the formation of butanol and total ABE-solvents using four different ABE-producing strains of clostridia: C. acetobutylicum ATCC 824, C. beijerinckii ATCC 55025, C. beijerinckii BA 101 and C. beijerinckii NCIMB 8052.