A combined system of size exclusion chromatography, UV absorption detection and ICP-MS was used for elucidation of the dissolved states of yttrium and lanthanide elements in lake water collected from Lake Biwa, Japan. The lake water samples were filtered with a membrane filter (pore size 0.45 µm) just after filtration with a glass filter (pore size 1.0 µm). The total concentrations of the dissolved lanthanide elements in the filtrate were determined by ICP-MS after chelating resin preconcentration. For speciation analysis, the above filtered water samples were further preconcentrated 30–500-fold with an ultrafiltration filter, which allowed preconcentration of molecules with molecular masses >10000. The preconcentrated sample solution was then subjected to speciation analysis using the above combined system, where a size exclusion column with the molecular permeation range between 1000 and 300000 was used for molecular separation. In the size exclusion chromatograms obtained with UV absorption detection at 254 nm, two peaks of some large organic molecules were obtained at retention times corresponding to molecular masses of >300000 (peak 1) and about 50000–10000 (peak 2). In the chromatograms, which were measured on-line by ICP-MS, yttrium and lanthanide elements (Y, La, Ce and Pr) were found at the two peak positions corresponding to above large organic molecules.