The influence of hydraulic retention time (HRT) and gelatin concentration on the acidification of gelatinaceous wastewater in an upflow anaerobic reactor was investigated at pH 5.5 and 37 o C. The degree of gelatin degradation increased with the HRT, from 84.1% at 4 h to 89.6% at 24 h, but decreased with the increase of the gelatin concentration in the influent from 65.2% at 2 g-CODl - 1 to 51.9% at 30 g-CODl - 1 . The degradation of gelatin followed the Monod kinetics with a maximum rate of 1.10 g (g-VSS.d) - 1 and a half-rate constant of 0.23 gl - 1 . The overall production rate of VFA and alcohols decreased with HRT, from 0.33 g (g-VSS.d) - 1 at 4 h to 0.15 g (g-VSS.d) - 1 at 24 h, but increased with gelatin concentration in the influent, from 0.10 g (g-VSS.d) - 1 at 4 g-CODl - 1 to 0.58 g (g-VSS.d) - 1 at 30 g-CODl - 1 . The key acidification products were acetate, propionate and butyrate, plus i-butyrate, valerate, i-valerate, caproate and ethanol in smaller quantities. Formate, methanol, propanol and butanol were found only in certain runs. Only 4.5-7.8% of COD in wastewater was converted to hydrogen and methane. The sludge yield was estimated as 0.320+/-0.014 g-VSS (g-COD) - 1 .