A process for producing ethanol from lactose containing substrates, comprising simultaneously saccharifying the substrate to produce monosaccharide and fermenting the monosaccharide to produce ethanol at a pH from 3.5-5.5, using a fermenting organism, wherein saccharification is carried out in the presence of a lactase, and wherein the fermenting organism is a Saccharomyces sp., and the ratio between the incubation time required for obtaining at least 90% hydrolysis of the lactose present in the substrate (t1) and the total fermentation time (t2) is in the range of 0.1 to 1, and the Saccharomyces sp. is added in amounts that will result in an ethanol yield of at least 70% w/w of the theoretical ethanol yield from lactose by the end of fermentation.