Total (TWW) and tapioca starch wash wastewater (TSWW) from a cassava processing plant in Thailand were analyzed for their composition with a view to evaluate their potential as substrates for solvent production by ABE fermentation with Clostridium spp. Starch was detected at a 1.63-fold higher level in the TWW than that in the TSWW (24.4% and 15.0% (w/w), respectively). The chemical oxygen demand (COD) was broadly similar (20,093 and 20,433 mg/L), but the biological oxygen demand (BOD) was 1.84-fold higher (18,000 and 9,750 mg/L) in the TWW than that in the TSWW. Thus, the TSWW was selected as a substrate to evaluate its potential for butanol and ethanol production by two Clostridium spp. The combined ethanol plus butanol production in the TSWW at pH 6.5 was higher than that at pH 4.5, being around 27.8- and 3.4-fold higher in C. butyricum TISTR 1032 and C. acetobutylicum ATCC 824, respectively. In both strains, the butanol (and combined butanol plus ethanol) production level was improved at pH 5.5. The addition of yeast extract increased the bacterial cell production, but did not significantly improve solvent productivity in C. acetobutylicum, and even decreased butanol production by C. butyricum.