In order to determine whether the 2PE emission of uranyl species could be used in microscopy the emissive properties of various uranyl(VI) containing species were studied in solution at ambient temperature, using a femtosecond-pulsed Ti:Sapphire laser with an operating range of 690 -1040 nm (full details of sample preparation and experimental set-up can be found in the ESI). Care was taken to choose concentrations of U VI O2 2+ , ligands, counterions, and solvents that would result in simple speciation profiles. For initial studies, the compounds used by [UO2Cl4] 2-and [UO2(NO3)3] -, were chosen as model systems. These species are known not to dominate aqueous systems but have been observed in acetonitrile. Solutions were prepared according to literature precedent and the one-photon excitation (1PE) spectra were obtained to identify the peak excitation wavelengths for each species. Two photon excitation experiments were then carried out at double these wavelengths. Gratifyingly, each species showed strong characteristic uranyl(VI) luminescence with the associated 'fingerprint' (due to strong vibronic coupling of the Raman active total symmetric stretch with the uranyl(VI) excited state). The emission profiles that resulted from 1PE and 2PE (Fig. ) were very similar, with any subtle differences likely arising from the different spectrometer resolution (for full experimental details see ESI). The shoulders seen in the emission spectrum of Fig 1a are attributable to further vibrational coupling with equatorially coordinated chloride ligands, as has been observed previously. A log-log plot of power against emission intensity demonstrated a quadratic power dependence (ESI), verifying that it was the result of 2PE. Excitation spectra were obtained over a range of 730 to 1040 nm (Fig. ). [UO2Cl4] 2-showed a simplified vibronic structure as described by Denning and although the resolution was lower, each of the major transitions reported in the solid state at <5 K were able to be assigned in the room temperature, solution state measurements. In the case of [UO2(NO3)3] -this is masked by vibrational broadening.