Formation and properties of UPD Cu deposits on polycrystalline Pt electrodes have been studied in sulfuric acid with various additions of chloride, thiourea (TU), acetonitrile (ACN) and polyethylenglycole (PEG). The experimental set-up combines coulometry, in situ microgravimetry and in situ differential reflectance spectroscopy. This set-up allows the simultaneous examination of the same electrode by recording cyclic voltammograms, frequency shifts of a mass sensitive oscillating quartz crystal and the reflected light intensities of a coherent light source from the surface. The latter technique uses a He-Ne laser which operates at a wavelength of 543 nm and was focused perpendicularly onto the Pt surface. The observed surface coverages theta, the electrosorption valencies gamma, and the optical properties Delta I/I of UPD copper adsorbates on Pt shed some light on their charge and size distribution and the adsorbate-substrate interaction.