In some reactions a "third body" is required for the reaction to proceed; this is often the case in dissociation or recombination reactions, such as

(3–18)

When a third body is needed, the concentration of the effective third body must appear in the expression for the rate-of-progress variable. Accordingly, the rate-of-progress variable is different from Equation 3–4 by the first factor in the equation

(3–19)

If all species in the mixture contribute equally as third bodies, then for all , and the first factor is the total concentration of the mixture,

(3–20)

However, it is often the case that some species act more efficiently as third bodies than do others. The that differ from 1 must be specified by auxiliary input to the Gas-phase Kinetics Pre-processor, as described in Problems Having No Reactions of the Chemkin Input Manual .