Structure transitions between copper UPD adlayers on Au(111)-($1 \times 1$) in sulfuric acid and chloride containing electrolyte were investigated by in situ scanning tunnelling microscopy. We demonstrate that co-adsorbed sulphate ions in the ($\sqrt{3} \times \sqrt{3}$)R30° UPD adlayer are replaced by chloride ions and, depending on the halide coverage, a commensurate ($2 \times 2$) or a slightly distorted ($5 \times 5$)-like Cu-Cl UPD adlayer are formed. The stability ranges of these phases are controlled both by the electrode potential and the Cl^- concentration. Phase transitions between the three UPD phases were monitored by time-resolved in situ STM. The observed structure details were attributed to mechanisms based on two-dimensional nucleation and growth processes.