Gas-phase reactions describe interactions of gas-phase species. Each species in a reaction must be associated with thermodynamic data. The thermodynamic data are used to calculate equilibrium constants and reverse-rate coefficients for a reaction. In this way the chemical rate expressions build on the thermodynamic expressions discussed in the previous chapter. Ansys Chemkin provides the user with a wide array of options for expressing gas-phase chemical reactions, where reaction rates can depend on species composition, temperature, and pressure. While elementary reactions that obey the law of mass action are the default formulation, the user has available a variety of optional formulations for specifying global or lumped rate expressions. In this chapter, we begin here by describing the more common elementary-reaction relations and then discuss the exceptions and user options in subsequent sections.