<P><B>Abstract</B></P> <P>Batch studies are used to benchmark biohydrogen potential (BHP) and biomethane potential (BMP) yields from feed substrates, digestates residues and different process configurations. This study shows that BMP yields using cellulose can be biased positively by not diluting the initial sewage sludge inoculum and the bias is independent of starting inoculum volatile solids (VS) concentration. The carryover of BHP inoculum also increased the BMP yields when using cellulose as a substrate by up to 18.8%. Furthermore it was also observed that the dilution of BMP inoculum with deionised H<SUB>2</SUB>O reduced methane yields from cellulose by up to 132±26NmL-CH<SUB>4</SUB> g-VS<SUP>−1</SUP>. Therefore it is proposed that inoculum and standard substrate controls (as used in this study) should be included in methane batch methodologies, particularly when using a pre-fermentation stage such as dark fermentation.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Total methane yield from cellulose was influenced by the inoculum volume. </LI> <LI> Diluted inoculum led to decreased methane yields from cellulose standards. </LI> <LI> Inoculum volume influences batch comparisons of single and two stage biogas yields. </LI> <LI> Inoculum carryover from pre-fermentation raised batch methane yields from cellulose. </LI> <LI> Batch methane yields should include substrate control values for different inoculums. </LI> </UL> </P>