  Chemical reactions often proceed through the formation and the consumption of
intermediate species. An example is the creation and subsequent degradation of
the substrate-enzyme complexes in an enzymatic reaction. In this paper we
provide a setting, based on ordinary differential equations, in which the
presence of intermediate species has little effect on the overall dynamics of a
biological system. The result provides a method to perform model reduction by
elimination of intermediate species. We study the problem in a multiscale
setting, where the species abundances as well a the reaction rates scale to
different orders of magnitudes. The different time and concentration scales are
parameterised by a single parameter $N$. We show that a solution to the
original reaction system is uniformly approximated on compact time intervals to
a solution of a reduced reaction system without intermediates and to a solution
of a certain limiting reaction systems, which does not depend on $N$. Known
approximation techniques such as the theorems by Tikhonov and Fenichel cannot
readily be used in this framework.
