The equilibrium solubility of uranyl nitrate in supercritical fluid carbon dioxide (SF-CO2) in the absence and presence of different modifiers was investigated. In the presence of methanol and HNO3, uranyl nitrate showed a considerable solubility in SF-CO2. SF-CO2 modified with different co-extractants such as tributylphosphate (TBP), methylisobutyl ketone (MIBK), acetylacetone (AA) and methanol (MeOH) at 60 degreesC and 250 atm was used for the extraction of uranyl nitrate from cellulose-based filter papers. The extraction efficiency of the system for uranyl ion was found to decrease in the order TBP > MIBK > AA > MeOH. The use of eight potential chelating agents bis(2-ethylhexyl)hydrogen phosphate (HDEPH), tri-n-octylphosphine oxide (TOPO), dicyclohexyl-18-crown-6 (DC18C6), dibenzoylmethane (DBM), 8-hydroxyquinoline (HOX), diphenylaminesulfonic acid (DPASA) 1,4-bis-[4-methyl-5-phenyl-2-oxazolyl] benzene (DMPOPOP) and TBP for uranyl ion extraction from filter paper samples by methanol modified SF-CO2 was investigated. The organophosphorous reagents TOPO and HDEPH showed the highest extraction efficiency in the series. The synergistic extraction of uranyl nitrate with HDEPH and TOPO in the presence of DC18C6 was also studied. (C) 2001 Elsevier Science B.V. All rights reserved.