We report the influence of two structurally related heterocyclic thiols, 2-mercaptobenzothiazole and 2-mercaptobenzimidazole, on the underpotential (upd) and bulk deposition of Cu on Au(111). X-ray photoelectron spectroscopy analysis of the organic adlayers reveal that after self-assembly from the liquid phase, the primary adsorbate species is consistent with a thiolate. The Au(111)-(root 3 x 22) reconstruction is lifted to the (1 x 1) phase upon thiol self-assembly. At submonolayer thiol coverages, a new upd feature is observed, with an intensity that goes through a maximum at intermediate thiol coverages. This feature is consistent with Cu adsorption in the vicinity of chemisorbed thiol. A thiol coverage-dependent study of Cu upd suggests formation of small two-dimensional thiol islands. At saturation coverage, the adsorbates passivate the surface towards Cu upd and significantly hinder bulk processes. Investigation of a previously reported potential-dependent reorientation of 2-mercaptobenzothiazole disputes a structural mechanism of ring ＇opening＇ and ＇closing＇ of the thiol layer. Instead, we suggest that instability of the thiol during cycles of Au oxide formation and reduction results in irreversible desorption of the thiol and complete restoration of Cu upd.