Detailed understanding of the respective roles of s olution and surface parameters on the reactions at uranyl solution / Al-(hydr)oxide inter faces is crucial to model accurately the behaviour of U in nature. We report studies on the effects of the initial aqueous uranyl speciation in moderately acidic solutions, e.g. of mononuclear, polynuclear uranyl species and / or (potential) U(VI) colloids, on the sorption of U by large surface areas of amorphous Al- hydroxide. Investigations by Extended X-ray Absorption Fine Structure (EXAFS) and Time- Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) reveal similar U coordination environments on Al-hydroxide for low to moderate U loadings of sorption samples obtained at pH 4-5, independently of the presence of mononuclear or polynuclear aqueous species, or of the potential instability of initial solutions f avoring true U-colloids formation. EXAFS data can be interpreted in terms of a dimeric, bidentate , inner-sphere uranyl surface complex as an average of the U surface structures. TRLFS data, ho wever, provide valuable insights into the complex U surface speciation. They indicate multipl e uranyl surface species under moderately acidic conditions, as predominant mononuclear and / or dinuclear, inner-sphere surface complexes and as additional minor species having U atoms in a uranyl (hydr)oxide - like coordination environment. The additional species pr obably occur as surface polymers and / or as adsorbed true U colloids, depending on the aqueo us U concentration level (1 to 100 µM).