The rates of ligand exchange into the geochemically important [NpO2(CO3)(3)](4-) aqueous complex are measured as a function of pressure in order to complement existing data on the isostructural [UO2(CO3)(3)](4-) complex. Experiments are conducted at pH conditions where the rate of exchange is independent of the proton concentration. Unexpectedly, the experiments show a distinct difference in the pressure dependencies of rates of exchange for the uranyl and neptunyl complexes.