Butanol production by four different Clostridium sp. strains was investigated using glucose P2-medium supplemented with increasing concentrations of butyric acid, added as cosubstrate. Batch fermentations were carried out in serum bottles (freely-suspended cell cultures) and fibrous-bed bioreactor (FBB) with medium recirculation (immobilized cells). Butyric acid clearly revealed to inhibit cellular growth with all specific growth rates declining upon the increase of butyrate concentrations. However, the presence of low and moderate levels in the medium can readily enhance the ABE-fermentation and increase butanol production through a shift induction towards the solventogenic phase controlled by the medium pH. In all cases it was found that 4.0 g⋅l-1 is the optimal concentration of butyrate that maximizes the yields for all ABE-solvents and butanol productivities. The non-mutant C. acetobutylicum ATCC 824 was singled out as the most efficient butanol productive strain among all bacteria tested (10.3 g⋅l-1 butanol versus 0.72 g⋅l-1 with and without 4.0 g⋅l-1 butyrate, respectively) showing a productivity augment in the order of 0.078 g⋅l-1⋅h-1 (78.5%) and yields of 0.3 g⋅g-1 from substrate and 7.6 g⋅g-1 from biomass versus 0.072 g⋅g-1 and 0.41 g⋅g-1 with and without the optimal butyrate concentration, respectively. This strain also revealed the best overall tolerance over increasing butyrate concentrations up to ∼6.0 g⋅l-1 and the highest glucose uptake (65.5%) among all bacteria. Furthermore, the beneficial effects of butyric acid were also observed through the use of a fibrous bed-bioreactor when the mutated strains of C. beijerinckii ATCC 55025 and BA 101 were tested. The use of this immobilized cell system effectively improved butanol production over the free system with butanol titers in the fermentation broth around 11.5 g⋅l-1 and 9.4 g⋅l-1 for the two bacteria, respectively, roughly doubling the values attained with the corresponding suspended cell cultures when the media were supplemented with 4.0 g⋅l-1 of butyrate. All these results confirm the enhancement of butanol formation using either free or immobilized cell cultures supplemented with butyric acid concentrations up to 4.0 g⋅l-1 in the media.