Underpotential deposition(UPD) of Zn(2+)ions on polycrystalline platinum in KH2PO4 solution (pH 4.6) was studied by Fourier transform infrared reflection spectroscopy (FTIR) and electrochemical quartz crystal microbalance (EQCM). By the addition of Zn2+ ions in KH2PO4 solution, the intensities of ir bands of around 990, 1044 and 1090 cm(-1) are increased with a negative increase of potentials at E less than or equal to 0.7 V (the) where Zn UPD takes place, while by EQCM the weight increase of Pt mounted quartz was observed in the same potential region. Coverage of UPD Zn was 0.75 at 0.05 V as two-electron transfer UPD process, being estimated from cyclic voltammogram, and mps (mass number per platinum sites) was 78 by EQCM; from both values the coverage of phosphate was evaluated as 0.27. These results are discussed that the phosphate species adsorbs over UPD Zn on Pt as a form of HPO4, where one of three P-O's seems to be bound to UPD Zn on Pt.