We studied the competitive chloride/MPS adsorption on Cu(100) by means of in situ STM. MPS (mercaptopropane sulfonic acid) spontaneously forms on the bare metallic copper surface by dissociative adsorption of SPS (bis-(3-sulfopropyl)disulfide). MPS self-assembles on Cu(100) giving rise to the formation of a c(2 x 6)-MPS adlayer with a nominal surface coverage of theta = 0.33 ML. Subsequent chloride co-adsorption from a mixed Cl/SPS containing electrolyte leads to the reorganization of the surface structure. The resulting co-adsorption phase can be rationalized in terms of a defect-rich c(2 x 2)-MPS/Cl adlayer with a nominal surface coverages of theta(Cl) = 0.25 ML and theta(MPS) = 0.25 ML. In the STM experiment visible, however, is a pattern with p(2 x 2) symmetry that corresponds to the terminating layer of sulfonate head groups of the MPS sub-lattice in the c(2 x 2)-MPS/Cl co-adsorption phase. The particular ratio of the MPS/Cl surface concentrations is proven to be dependent on the ratio of the SPS/Cl concentrations in the bulk solution. The present STM work demonstrates quasi-reversibility in the MPS/Cl surface phase behavior. (C) 2013 The Electrochemical Society. All rights reserved.