A labeled goethite was synthesized, using oxygen‐stable isotope18O, in order to reveal ligand exchange reactions accompanying selenite sorption on the particles. The18O isotope ratio of the surface hydroxyl groups on the labeled goethite was estimated to be 3.2% or 4.8% (depending on the assumed cases) by measuring the increase in the18O isotope ratio of water molecules of the solutions through fluoride sorption on the goethite. The amount of oxygen atoms from surface hydroxyl groups on the labeled goethite released by selenite sorption was estimated, approximately, by the measurement of changes in the oxygen isotope ratio of water molecules in the reaction solutions. The δ18O value of the selenite sorptive solutions was apparently increased, suggesting that ligand exchange reactions occurred between selenite ions and the surface hydroxyl groups on the labeled goethite. The results suggested that the ligand exchange reactions of selenite sorption consisted of 6 to 40% of reaction in which oxygen atoms of the surface hydroxyl groups were released from the particles and of 60 to 94% of the alternative reaction in which oxygen atoms of selenite ions were released into the solutions, if the reaction sites for selenite sorption on the goethite are the samcas for fluoride sorption.