Complex reaction schemes in heterogeneous, homogeneous or enzymatic catalysis often contain reaction routes, which do not result in chemical transformations. Existence of these empty routes imposes thermodynamic constraints on the values of kinetic parameters, e.g. rate coefficients in the steps, which constitute the empty routes. Thermodynamic analysis is presented in connection with reaction of 7,8-dihydrofolate and NADPH to form 5,6,6,8-tetrahydrofolate and NADP, catalyzed by dihydrofolate reductase. (c) 2007 Elsevier Ltd. All rights reserved.