It is demonstrated by using in situ scanning tunneling microscopy (STM) that well-defined Ir(lll) surfaces can be exposed in aqueous solutions by a flame-annealing-quenching technique. Atomically flat terrace-step structures were consistently observed in the double layer potential range in an aqueous HClO4 solution. It is shown that sulfate/bisulfate anions form highly ordered adlayers on Ir(lll) in H2SO4 solution over a wide potential range. In situ STM images disclosed an Ir(111)-(root 3 x root 7) structure, which is identical with that found previously on Au(lll), Pt(lll), Rh(111), and Cu(lll). High;resolution STM imaging allowed us to determine the orientation of hydrogen-bonded water molecules. Two different water molecules were clearly distinguished as separate spots along the root 3 direction between neighboring rows of adsorbed sulfate.