Interaction of dioxouranium(VI) (uranyl) ion with ATP was studied by ligand/proton and metal/hydroxide displacement technique, at very low ionic strength and at I=0.15 mol L(-1), in aqueous Me4NCl and NaCl solutions, at t=25 degrees C. Measurements were carried out in the pH range 3-8.5, before the formation of precipitate. Computer analysis allowed us to find the quite stable species UO2(ATP)H2(0), UO2(ATP)H-, UO2(ATP)2-, UO2(ATP)2(6-), UO2(ATP)2H2(4-) and UO2(ATP)(OH)3- whose formation constants are (at I=0 mol L(-1)) logbeta(112)=18.21, logbeta(111)=14.70, logbeta(110)=9.14, logbeta(120)=12.84, logbeta(122)=24.82, and logbeta(11-1)=2.09, respectively. Different values were obtained in the above ionic media at I=0.15 mol L(-1) and the dependence on the ionic medium was interpreted in terms of interactions between the negatively charged complex species and cations of supporting electrolytes. The species more stable in NaCl than in Me4NCl are those with the highest negative charge, UO2(ATP)2(6-) and UO2(ATP)2H2(4-), and the extra stability of these species can be attributed to the interaction with Na+. Speciation profiles show that ATP can suppress UO2(2+) hydrolysis, and that in the neutral to slightly alkaline range the yield of complex UO2-ATP species is quite high. Comparison with other metal-ATP systems is also given in order to recognize the possibility of binding competition of uranyl ion in metal-ATP requiring enzymes for biochemical processes.