  We present a new method to analyse and reduce chemical networks and apply
this technique to the chemistry in molecular clouds. Using the technique, we
investigated the possibility of reducing the number of chemical reactions and
species in the UMIST 95 database simultaneously. In addition, we did the same
reduction but with the ``objective technique'' in order to compare both
methods. We found that it is possible to compute the abundance of carbon
monoxide and fractional ionisation accurately with significantly reduced
chemical networks in the case of pure gas-phase chemistry. For gas-grain
chemistry involving surface reactions reduction is not worthwhile. Compared to
the ``objective technique'' our reduction method is more effective but more
time-consuming as well.
