The voltammetry of the formation of(root 3 x root 3)R30 degrees and p(2 x 2) overlayers on (111) electrodes is modeled by analytical and Monte Carlo techniques. Both ordered structures are formed by second-order order-disorder phase transitions that lead to sharply-peaked 'butterfly' features in the voltammogram. The butterflies for both systems are, however, distinctly different and resemble the voltammetry of Pt(lll) in sulfuric and perchloric acid, respectively, even though the simulated adlayer structures are not exactly the same as the experimental ones. Some general features of butterfly peaks in voltammetry and their implications are discussed.