Detailed balance states that in equilibrium each elementary process is equilibrated by its reverse process and requires reversibility of all elementary processes. For many real physico-chemical complex systems (e.g. homogeneous combustion, heterogeneous catalytic oxidation, most enzyme reactions etc.), detailed mechanisms include both reversible and irreversible reactions. If one represents irreversible reactions as limits of reversible steps, then it becomes obvious that not all reaction mechanisms with irreversible reactions can be obtained as limits of systems or reversible reactions with detailed balance. For example, the irreversible cycle A 1 ⟶ ⟶ A 2 ⟶ ⟶ A 3 ⟶ ⟶ A 1 cannot be obtained as such a limit but the reaction mechanism A 1 ⟶ ⟶ A 2 ⟶ ⟶ A 3 ⟵ ⟵ A 1 can.