This paper deals with the development, analysis and reduction of a two-stage anaerobic digestion process model. A two-stage model based on a two-step mass-balance model is first developed. The model incorporates two stages as two similar CSTRs, with acidogenesis in the first stage and mainly methanogenesis in the second. Steady state optimization validates the benefits of two-stage operation compared to the traditional one-stage process. Using reaction invariants, the dynamics of the fast modes is neglected by projecting the overall system dynamics on the slow-motion invariant manifold, and the original 7-state model is reduced to a 5th order system. By neglecting the nearly unobservable modes, the model is further reduced to 3rd order. Simulation results show excellent accuracy of the reduced model compared to the detailed one. At the same time, they indicate the necessity of process control for stabilization of the system. (C) 2017 Elsevier B.V. All rights reserved.