Chemical reaction systems of practical interest are usually very complex: They consist of a large number of elementary reactions (hundreds or thou sands in a small system), mostly with rate coefficients differing by many orders of magnitude, which leads to serious stiffness, and they are often coupled with surface reaction steps and convective or diffusive processes. Thus, the derivation of a "true" chemical mechanism can be extremely cumbersome. In most cases this is done by setting up "reaction models" which are improved step by step using, for example, perturbation theory, numerical...