Uranyl ion can be separated from aqueous solution and preconcentrated on XAD-4 modified with organic reagents. Dibenzoyl methane (DBM), 8-hydroxy quinoline (oxine), and tri-n-octyl phosphine oxide (TOPO) were impregnated on the adsorber resin and treated with aqueous solutions containing microgram quantity of uranium. The preconcentrated uranium could be eluted out with dilute acid from the column modified with DBM or oxine, while ammonium bicarbonate was necessary for the elution of uranium from the column modified with TOPO. Uranium in the eluate was determined spectrophotometrically using Arsenazo(III). Adding EDTA to the sample solution could separate uranium from transition metal ions and thorium. The method of preconcentration has been successfully applied to the recovery of uranium from synthetic seawater.