Butyric acid is a valuable chemical that has various applications in the chemical, pharmaceutical, food, and biofuel industries. Its bio-fermentation from lignocellulosic materials may be affected by some unwanted substrates that are generated during biomass pretreatment processes. In the present study, the potential inhibitory effects of two phenolic aldehydes (syringaldehyde and vanillin) on butyric acid fermentation by Clostridium tyrobutyricum were evaluated. The toxicity of syringaldehyde and vanillin on cell growth, xylose consumption, and butyric acid production was dosage-dependent. The butyric acid productivity decreased significantly with increasing concentrations of syringaldehyde and vanillin. C. tyrobutyricum demonstrated a broad tolerance to both syringaldehyde and vanillin and only moderate reductions in the maximum cell density were observed with up to 2.4 g L-1 of syringaldehyde or vanillin in the medium. Both syringaldehyde and vanillin were assimilated by C. tyrobutyricum, and the metabolite products from vanillin caused considerable inhibition of the fermentation.