The IWA Anaerobic Digestion Model No.1 (ADM1) has been extensively used in recent years. However, its application to non-methanogenic systems is limited by the use of constant-stoichiometry to describe product formation from carbohydrate fermentation. This study presents a modification of the ADM1 using a variable stoichiometry approach, derived from experimental information. The biomass and product yields from glucose degradation are assumed to be dynamically depending on the total concentration of undissociated acids in the reactor. Experimental data from an 11 L mesophilic continuous bio-hydrogen reactor fed with 20, 40, 50 and 10 g/L of sucrose, were used to validate the approach. The modified model achieved good predictions of the experimental data, using the standard ADM1 parameter values, without any parameter fitting beyond the implementation of the variable stoichiometry. The modification approach proposed extends the applicability of the ADM1 to non-methanogenic fermentative systems and in particular to continuous bio-hydrogen production.