Ab initio calculations are used to analyze the interaction between formic acid and CH3X, for X equal to each of F, Cl, and Br. All minima are cyclic in that they contain more than one H-bond. The most strongly bound contain a OH...X bond along with CH...O, and the others contain CH...X and CH...O interactions. Alterations of the covalent bond lengths within each subunit, and vibrational frequency shifts, coupled with electronic charge shifts, reveal fundamental features of these complexes, and the nature of the interactions. The OH...X bond is the strongest of those examined here, followed by CH...X and CH...O. (c) 2006 Elsevier B.V. All rights reserved.