The possibility of shifting a methanogenic process for hydrogen production by changing the process parameters viz., organic loading rate (OLR) and hydraulic retention time (HRT) was evaluated. At first, two parallel semi-continuously fed continuously stirred tank reactors (CSTR) were operated as methanogenic reactors (M1 and M2) for 78 days. Results showed that a methane yield of 198-218 L/kg volatile solids fed (VSfed) was obtained when fed with grass silage at an OLR of 2 kgVS/m(3)/d and HRT of 30 days. After 78 days of operation, hydrogen production was induced in M2 by increasing the OLR from 2 to 10 kgVS/m(3)/d and shortening the HRT from 30 to 6 days. The highest H-2 yield of 42 L/kgVS(fed) was obtained with a maximum H-2 content of 24%. The present results thus demonstrate that methanogenic process can be shifted towards hydrogen production by increasing the OLR and decreasing HRT. (C) 2011 Elsevier Ltd. All rights reserved.