<P><B>Abstract</B></P> <P>Pilot tests of sequential dark and photo fermentation H<SUB>2</SUB> production were for the first time conducted in a 11 m<SUP>3</SUP> reactor (3 m<SUP>3</SUP> for dark and 8 m<SUP>3</SUP> for photo compartments). A combined solar and light-emitting diode illumination system and a thermal controlling system was installed and tested. With dark fermentation unit maintained at pH 4.5 and 35 °C and photo fermentation unit at pH 7.0 and 30 °C, the overall biogas production rate using hydrolyzed corn stover as substrate reached 87.8 ± 3.8 m<SUP>3</SUP>/d with 68% H<SUB>2</SUB> content, contributed by dark unit at 7.5 m<SUP>3</SUP>-H<SUB>2</SUB>/m<SUP>3</SUP>-d and by photo unit at 4.7 m<SUP>3</SUP>/m<SUP>3</SUP>-d. Large variation was noted for H<SUB>2</SUB> production rate in different compartments of the tested units, revealing the adverse effects of poor mixing, washout, and other inhomogeneity associated with large reactor operations.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Pilot tests of sequential dark and photo fermentation H<SUB>2</SUB> production were conducted. </LI> <LI> Reactors of size 11 m<SUP>3</SUP> reactor (3 m<SUP>3</SUP> + 8 m<SUP>3</SUP>) were used with corn stover as substrate. </LI> <LI> Biogas production rate 87.8 ± 3.8 m<SUP>3</SUP>/d with 68% H<SUB>2</SUB> content was achieved. </LI> <LI> Large variation in performance was noted in large-scale reactors. </LI> </UL> </P>