We present theoretical proof that the nature of the interactions existent in the complexes formed between hydrogen fluoride and a series of pi-systems (acetylene, ethylene, cyclopropene, cyclobutadiene, and benzene) and three-memebered-ring derivatives (cyclopropane and tetrahedrane) is that of a hydrogen bond between the hydrogen atom and the pi-cloud. Geometric and energetic data and mainly the study of the topology of the electronic density within the frame of the theory of atoms in molecules established by Bader have been used for this analysis.