We studied the influence of initial pH on hydrogen (H-2) production using permeate from scotta (a partially deproteinized cheese whey from ricotta production) as substrate (51 g L-1 lactose). Dark fermentation was carried out at 35 degrees C in laboratory batch reactors, in an unbuffered system. Hydrogen production and metabolite (volatile fatty acids, ethanol, and lactic acid) evolution during a 96-h period were monitored in reactors with initial pH varying in the range 4-10. In all reactors, H-2 production started only when pH fell below 6. However, it was much higher (+31%) in the reactors with initial alkaline pH. We conclude that H-2 production occurs only at acidic pH values, but initial alkaline pH values increase the overall H-2 production in dark fermentation of lactose-rich substrates. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.