TheGvalue refers to the number of molecules of reactant consumed or product formed per 100 eV of energy absorbed. Although ubiquitous in radiation chemistry, theG‐value concept appears only rarely in plasma chemistry. We derive a formula for theGvalue for a general plasma chemical reaction as a function of the electrical power absorbed, the fraction of molecules transformed, and the flow rate of the gas entering the reactor. Applying our formula to the ammonia plasma radiofrequency discharge data of d’Agostinoetal. [Plasma Chem. Plasma Process.1, 19 (1981)], we find thatG(‐NH3) depends in general upon the conditions but lies in the range of 6.0–20 molecules/100 eV. By comparison, the values ofG(‐NH3) reported by Peterson for the gas‐phase radiolysis of ammonia lie in the range 2.7–10 molecules/100 eV. We suggest that this similarity with respect to order of magnitude may have its origin in a common reaction mechanism initiated by inelastic electron‐molecule collisions. We use this hypothesis and the Boltzmann transport theory to derive a formula which expressesG(‐NH3) in terms of electron‐molecule scattering cross sections, the distribution of electron velocities, and the rate constants for secondary reactions. In principle, given a knowledge of the reaction mechanism and the electron‐energy‐loss channels, our method may be used to express theGvalue of any gas.