The potential-dependent (2×2)-3CO to a (√19×√19)R23.4°-13CO adlayer phase transition on Pt(111) with 0.1M H2SO4 electrolyte was studied using femtosecond broadband multiplex sum frequency generation (SFG) spectroscopy combined with linear scan voltammetry. Across the phase boundary the SFG atop intensity jumps, and at the same time the SFG spectrum of threefold CO sites is transformed into a bridge site spectrum with a small decrease in integrated SFG intensity. This occurs because the SFG signal is sensitive to both the coverage and changes in the local field that result from a changing adlayer structure. The phase transition is reversible with an Δ150 mV anodic overpotential and the forward (2×2)-3CO to a (√19×√19)R23.4°-13CO transition is slower than the reverse. Repeated cycles of phase transition indicate that the 25 μm electrolyte layer used here does not appreciably distort the potential-coverage relationships. The most recent data obtained on the electrode potential dependent SFG characteristics of the Pt(111)/Ru-CO system will be reported.