The behaviour of the chemical affinity at the stage of intermediate product formation was analysed in a monomolecular, a homogenous catalytic and in a nucleophilic substitution reactions. Under conditions of the steady-state approximation, the rate of change of the chemical affinity at this stage of reaction must be such that the rate of intermediate formation and that of its decay are nearly identical. This assumption was used to determine a quantitative criterion for the steady-state approximation. The method used to derive a criterion is also given. The examples of applications were provided.