The adsorption of hydrogen adatoms and bisulfate anions on an Ir(111) electrode surfaces in 0.5 M H2SO4 solution has been investigated in situ with a combination of cyclic voltammetry (CV), infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) experiments. The IRAS result shows evidence for the adsorption of hydrogen adatoms occupying the on-top sites of the Ir(111) at potentials below 55 mV versus SHE (standard hydrogen electrode) where current spikes are seen in the CV profile. The STM reveals a (1 x 1) structure corresponding to the unreconstructed Ir(111) lattice at potentials below 55 mV. At potentials positive of the current spikes, two absorption bands at 1255-1320 and 945-956 cm(-1) are present in the IR spectra with the former bands showing a higher frequency shift upon deuteration. The IRAS and STM results demonstrate that HSO4- anions rather than SO42-anions predominantly adsorb on the Ir(111) electrode to form the characteristic (root3 x root7) structure.