Abstract. The chemical master equation plays a fundamental role for the understanding of gene regulatory networks and other discrete stochastic reaction systems. Solving this equation numerically, however, is usually extremely expensive or even impossible due to the huge size of the state space. Thus, the chemical master equation must often be replaced by a reduced model which operates with a considerably smaller number of degrees of freedom but hopefully still provides the essential information about the dynamics of the full system. We prove error bounds for two reduced models which have previously been proposed in the literature. Based on the error analysis, an alternative model reduction approach for the chemical master equation is introduced and discussed, and its advantage is illustrated by numerical examples.Key words. Chemical master equation, model reduction, hybrid models, error bounds AMS subject classifications. 34K60, 65C20, 60J27, 92D251. Introduction. Many processes in nature can be considered as reaction systems in which d ∈ N different species interact via r ∈ N reaction channels. The time evolution of such a system is usually modeled by the traditional reaction-rate equations, a set of d coupled ordinary differential equations which indicate how the concentrations of the d species change in time. This approach is simple and computationally cheap, but fails in situations where the influence of inherent stochastic noise cannot be ignored, and where certain species have to be described in terms of integer particle numbers instead of real-valued, continuous concentrations. This is the case in gene regulatory networks, viral kinetics with few infectious individuals, and many other biological systems.The chemical master equation (CME) respects the discreteness and randomness of the problem and thus provides a more accurate model. The system is considered as a random variable Z(t) which evolves according to a Markov jump process on N