  We derive a reduction formula for singularly perturbed ordinary differential
equations (in the sense of Tikhonov and Fenichel) with a known parameterization
of the critical manifold. No a priori assumptions concerning separation of slow
and fast variables are made, or necessary.We apply the theoretical results to
chemical reaction networks with mass action kinetics admitting slow and fast
reactions. For some relevant classes of such systems there exist canonical
parameterizations of the variety of stationary points, hence the theory is
applicable in a natural manner. In particular we obtain a closed form
expression for the reduced system when the fast subsystem admits complex
balanced steady states.
