The tautomeric process of neutral formamide, HC(OH)NH→HCONH 2, catalysed by one neutral molecule (H2O, NH3 and HF) and by a proton-exchanged zeolite (HZ) has been studied by means of quantum chemical methods. The role of the catalyst in the tautomeric process depends on the relative acid-base character of the neutral molecule with respect to the formamide substrate. The analysis of the transition structures has allowed us to define two types of catalysis: the acidic and the basic bifunctional ones. The topological analysis of Bader "Atoms in Molecules" is performed in order to interpret the two types of catalysis. © 2003 Elsevier B.V. All rights reserved.