The objective of the study was to valorize waste stream for the co-production of hydrogen and caproate via open culture fermentation (OCF). Batch studies confirmed that the use of sugar (lactose) together with carboxylates (lactate and acetate) may allow mutual coexistence of chain elongation and dark fermentation processes. During the continuous test in an upflow anaerobic sludge blanket reactor (UASB), acid whey was used as a model feedstock due to a high concentration of lactose and lactate. Shortening hydraulic retention time (HRT) to 2.5 days allowed the co-production of hydrogen and caproate with almost complete methanogenesis inhibition. During the 50 days period, the average hydrogen and caproate production were 1.78 ± 0.75 L_H2/L/d and 133.4 ± 17.9 mmol C/L/d, respectively.