The Cu underpotential deposition (UPD) on Au(111) in the presence of sulfate ions is inarguably the most intensively studied UPD processes over the last two decades. This process is characterized by unique adsorption of sulfate ions that splits Cu monolayer formation in distinct energetic stages: (i) Formation of a low-density Cu phase also known as (��3 x ��3)R30 phase, at Cu coverage of 2/3 ML accompanied by adsorption of 1/3 ML of sulfate; (ii) Formation of a dense phase i.e. a pseudomorphic (1x1) Cu monolayer. Determination of the Cu UPD adlayer structure has not been easy and has not been without its fair share of controversy. In this work we present the most recent in situ Scanning Tunneling Microscopy (STM) study of the phase transition between the two deposition stages. Our findings address the still open and debated issue of sulfate adsorption on pseudomorphic (1x1) Cu layer. Previous SPM (STM and AFM) results reported imaging of Cu atoms in the dense phase thus implicating of no anion adlayer present on top of it. However, electrochemical results (EQCM and chronocoulometry), theoretical models, X-ray adsorption spectroscopy, in situ stress measurements, etc. all strongly support sulfate adsorption process. Our in situ STM results clearly show existence of an ordered sulfate structure on the pseudomorphic (1x1) Cu layer. A high-quality STM performance unambiguously reveals two ordered sulfate structures present on coexisting low- and high density Cu phases during the phase transition process.