Multi-phase anaerobic reactor for H-2 and CH4 production from paperboard mill wastewater was studied. The reactor was operated at hydraulic retention times (HRTs) of 12, 18, 24, and 36 h, and organic loading rates (OLRs) of 2.2, 1.5, 1.1, and 0.75 kg chemical oxygen demand (COD)/m(3) day, respectively. HRT of 12 h and OLR of 2.2 kg COD/m(3) day provided maximum hydrogen yield of 42.76 +/- 14.5 ml/g CODremoved and volumetric substrate uptake rate (-rS) of 16.51 +/- 4.43 mg COD/L h. This corresponded to the highest soluble COD/total COD (SCOD/TCOD) ratio of 56.25 +/- 3.3 % and the maximum volatile fatty acid (VFA) yield (Y-VFA) of 0.21 +/- 0.03 g VFA/g COD, confirming that H-2 was mainly produced through SCOD conversion. The highest methane yield (18.78 +/- 3.8 ml/g CODremoved) and -rS of 21.74 +/- 1.34 mgCOD/L h were achieved at an HRT of 36 h and OLR of 0.75 kg COD/m(3) day. The maximum hydrogen production rate (HPR) and methane production rate (MPR) were achieved at carbon to nitrogen (C/N) ratio of 47.9 and 14.3, respectively. This implies the important effect of C/N ratio on the distinction between the dominant microorganism bioactivities responsible for H-2 and CH4 production.