We measured the molar Peltier heat during the course of Cu underpotential deposition (UPD) in sulfate-containing solutions by electrochemical microcalorimetry. The molar Peltier heat during Cu UPD deviates considerably from that of Cu bulk deposition in the corresponding solution. Since the molar Peltier heat directly reflects the reaction entropy of the involved electrochemical reaction, including also charge-neutral side processes, this finding signals significant contributions of non-Faradaic side reactions. For the first stage of the Cu UPD this process is known to be coadsorption of Cu2+ and sulfate species. We retrieved the potential dependent surface coverages of Cu and sulfate during the first Cu UPD stage with the single additional assumption that the Cu coverage in the first stage of the Cu UPD reaches 2/3 ML. In addition, we found that both HSO4(ad) and SO4(ad) are adsorbed on the surface with a noticeable surplus of bisulfate. For the second stage of Cu UPD, i.e., the completion of the first (...