The surface reaction mechanism may consist of any number of chemical reactions involving the solid species named in the site and bulk data, as well as the gas-phase species declared in the Gas-phase Kinetics input file. A reaction may be reversible or irreversible. Reaction data must start with a line containing the word REACTIONS (or REAC). The lines following the REACTIONS line contain reaction descriptions together with their Arrhenius rate coefficients. The reaction description is composed of reaction data and perhaps optional auxiliary reaction data.