We have studied the electronic and structural properties responsible for the molecular recognition that the 1,3-diphenyl-thiourea ionophore (L) experiences by the Hg2+ cation. The theoretical data was obtained for the bare L and for the [L-HgOH](+) and [L-Hg-L](2+) coordination compounds involved in the reaction mechanism in reported sensors. Calculations were performed with the Gaussian-98 program at the B3LYP/6-31G** level. A LANL2DZ pseudopotential was used for Hg. The calculated electrostatic potential of the ionophore has a maximum on the S atom and, with less intensity, on the Z-phenyl group; consistently, high negative charges occur on those sites. The two highest occupied molecular orbitals are also mainly located on the sulfur-Z-phenyl atoms. Thus, this region defines the active site. These results account for the softness of the S atom that is bonded to Hg2+ in the complexes. The metal-sulfur interaction is key in the observed selectivity of Hg2+ by L. However, a significant Z-phenyl-Hg2+ bonding was also found. This rather unexpected result suggests that Z-phenyl is crucial in the recognition of Hg2+. In fact, in [L-H-L](2+), two S atoms and two Z-phenyl groups carry the Hg2+ ion.