Humic acid (HA) in the environment may exist in either dissolved or fixed forms. However, laboratory studies usually take only the former into account. Here we synthesized a hybrid of HA and aluminum hydroxide (Al(OH)(3)) to mimic fixed HA, compared the effects of fixed and dissolved HA on Eu(III)/Yb(III) adsorption on Al(OH)(3), and analyzed the adsorption mechanisms using time resolved laser induced fluorescence spectroscopy (TRLFS), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). It was found that dissolved HA affected significantly Eu(III)/Yb(III) adsorption on Al(OH)(3), whereas fixed HA showed little apparent effect on the adsorption. The spectra of TRLFS, XPS and EXAFS for Eu(III)/Yb(III) adsorption on Al(OH)(3) in the presence of dissolved/fixed HA could be reproduced by those for Yb(III)/Eu(III) adsorption in the absence of HA and those for Yb(III)/Eu(III) binding to dissolved HA, respectively. Spectroscopic analyses indicated that the different effects of fixed and dissolved HA on Eu(III)/Yb(III) adsorption were due to different surface speciation with the same surface species, which could be interpreted by the decrease of available sites on fixed HA as compared to those on dissolved HA. This study implied that the effects of HA on the adsorption of Eu (III)/Yb(III) as well as other trivalent lanthanides/actinides in the environment might be overestimated if the differences between dissolved and fixed HA were not considered.