Unlike the trivalent lanthanides, uranyl emission is not significantly quenched by the vibrational modes of surrounding solvent molecules. In alcoholic and, to a lesser extent, aqueous solutions the excited states can however be deactivated by redox processes. It was therefore important to confirm that 2PE could result in measurable emission in aqueous environments. Biologically relevant ligands were used to prepare solutions in which a single species would dominate, expressing a range of symmetries and coordination geometries -[UO2(H2O)5] 2+ , [UO2(CH3CO2)3] -, and [UO2DPA2] 2-. Pyridine dicarboxylic acid (also referred to as dipicolinic acid -DPA) was chosen as an example of a chelating ligand that is known to strongly bind U VI O2 2+ and is found in bacterial spores. In each case the emission showed the expected quadratic power dependence for 2PE (ESI), the slight deviations from the exact value are known to be the result of saturation effects. In all cases the vibronic structure was visible in both the 1PE and 2PE emission and excitation spectra. In the 2PE spectra this structure tended to be slightly red-shifted with respect to the corresponding peaks in the 1PE at approximately ½λ (Fig. ). This is likely due to the different selection rules for 2PE favouring different transitions. The ability of a molecule to undergo 2PE is quantified as its two-photon cross section, which is reported in Goeppert-Mayer (GM) units with 1 GM = 10 -50 cm 4 s photon -1 . These can be measured directly or by comparison to a dye standard, as was carried out here with Coumarin 485 chosen as the standard due to the proximity of the emission maximum with that of uranyl species. For each species two-photon absorption cross sections were obtained over a range of wavelengths to produce spectra that roughly corresponded with the two-photon excitation spectra (ESI). This represents the first time that the direct (non-sensitized) twophoton absorption cross section of an f-block metal has been recorded. Other work has looked at direct two-photon excitation of lanthanides, but not gone so far as to determine the cross section; 57-59 sensitised excitation of lanthanides has been investigated, reporting Tb(III) emission sensitised by tyrosine to have a cross sections of 7.4 and 0.4 GM at 503 and 566 nm, respectively. Operational two-photon dyes possess two photon cross section values ranging from single digits to the tens of thousands of GM. Compared to this, the U VI O2 2+ species studied here possess low two-photon cross sections (Table ). This is unsurprising, as the organic molecules for which these values are usually reported are designed to show structural features such as extended π-conjugated system and donor-π-acceptor-π-donor arrangements that are known to result in high cross sections. Table . Photophysical properties of the uranyl(VI) species used in this study. DPA = dipicolinic acid.