The kinetics of the exchange reaction between uranyl nitrate-organic phosphate complex and excess phosphate was studied by NMR line broadening. The organic phosphates were tributyl, dibutyl cresyl and discresyl butyl phosphate. The influence of solvents on rate constants was investigated, the order of the reaction was ascertained and activation energies were determined. Strikingly the activation energy was the same, within experimental accuracy for all three phosphates in all the solvents investigated 9 kcal/mole. Qualitative results were also obtained for the more bulky tri (Hexyl-2 ethyl) phosphate which was found to exchange at a lesser rate than the above mentioned phosphates. The problems arising when it is attempted to use the rate constants of the exchange reaction to estimate the permeability of uranyl nitrate through "solvent membranes" containing the same phosphates are discussed.