Presented herein is the experimental observation of the long-range ordered (root19 x root19) R23.4degrees-13CO structure on Pt(111) in aqueous electrolytes by in situ surface X-ray scattering. The results confirmed the presence of two mirrored domains suggested earlier on the basis of scanning tunneling microscopy and infrared measurements. Based on the weak intensity of the second order adlattice reflections and earlier results obtained by other techniques, a further refinement of the (root19 x root19) structure with tilted CO molecules is proposed. The hystereses observed in transitions between (2 x 2)- 3CO and (root19 x root19) R23.4degrees- 13CO phases, as well as in CO adsorption and stripping, with change in electrode potential are discussed. (C) 2004 The Electrochemical Society.