A topological analysis of the electron charge density in complexes between derivatives of squaric acid and ammonium cation has been carried out. There are excellent relationships between either the charge density or its associated Laplacian at the ring critical point originated upon complexation and both geometric and energetic parameters. Then is a remarkable influence of the substituents attached to the four-membered ring on the charge density at the ring critical point. (C) 2001 Elsevier Science B.V. All rights reserved.