Speciation of actinides, lanthanides, and fission products is a major challenge in the framework of nuclear fuel cycle studies. Electrospray–mass spectrometry has been used for uranium speciation. Free uranyl (UO22+), the first hydroxo complex (UO2OH+), and the oligomeric species (UO2)3(OH)5+ have been observed as a function of pH. Different aqueous solvents (sodium perchlorate, perchloric acid/ammonium acetate, and perchloric acid/ammonia) were tested in order to limit the presence of adducts. The influence of various parameters such as the cone voltage, temperature, and gas flow rate is presented. Comparison with results obtained by time-resolved laser-induced fluorescence on the same solutions shows a similar tendency despite a slight shift in pH relative to the OECD database.