A new way of calculating lost work in a reacting, diffusing mixture is presented. We show that a fast chemical reaction can be treated within the framework of coupled transport equations for heat and mass, after elimination of dependent components. Two approaches to reduction of lost work follow from the equations : reduction of driving forces and utilization of coupling. When Fick’s and Fourier’s laws apply, losses can be minimized, but not avoided, by reduction of driving forces. When transport processes are coupled, as expressed by the theory of irreversible thermodynamics, loss reductions can be obtained also through the coupling coefficients. Maximum reduction is obtained for strict coupling, i.e. unique flux relationships. Coupling implies physical interaction and is discussed for isothermal and non-isothermal systems. The relation between irreversible thermodynamics and exergy analysis is also discussed.