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 degrees 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 x 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 x d)(-1) at 4 h to 0.15 g (g-VSS x d)(-1) at 24 h, but increased with gelatin concentration in the influent, from 0.10 g (g-VSS x d)(-1) at 4 g-CODl(-1) to 0.58 g (g-VSS x 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).