A useful rule in the determination of mechanism is that the concentration factors in the rate law indicate the composition and charge of the activated complex or transition state. For the –CO reaction above, the rate depends on [], so that the activated complex has composition , with 2  entering the reaction before the transition state, and CO reacting after the transition state.
A multistep example is the reaction between oxalic acid and chlorine in aqueous solution:  +  → 2  + 2  + 2 .
The observed rate law is 
which implies an activated complex in which the reactants lose 2 +  before the rate-determining step. The formula of the activated complex is  +  − 2  −  + &thinsp;, or  (an unknown number of water molecules are added because the possible dependence of the reaction rate on  was not studied, since the data were obtained in water solvent at a large and essentially unvarying concentration).
One possible mechanism in which the preliminary steps are assumed to be rapid pre-equilibria occurring prior to the transition state is
: +   HOCl +  +    
:HOCl +  →  +  + 2