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The positive impact of having access to well-defined starting materials for applied actinide technologies - and for technologies based on other elements - cannot be overstated. Of numerous relevant 5f-element starting materials, those in complexing aqueous media find widespread use. Consider acetic acid/acetate buffered solutions as an example. These solutions provide entry into diverse technologies, from small-scale production of actinide metal to preparing radiolabeled chelates for medical applications. However, like so many aqueous solutions that contain actinides and complexing agents, 5f-element speciation in acetic acid/acetate cocktails is poorly defined. Herein, we address this problem and characterize Ac 3+ and Cm 3+ speciation as a function of increasing acetic acid/acetate concentrations (0.1 to 15 M, pH = 5.5). Results obtained via X-ray absorption and optical spectroscopy show the aquo ion dominated in dilute acetic acid/acetate solutions (0.1 M). Increasing acetic acid/acetate concentrations to 15 M increased complexation and revealed divergent reactivity between early and late actinides. A neutral Ac(H 2 O) 6 (1) (O 2 CMe) 3 (1) compound was the major species in solution for the large Ac 3+ . In contrast, smaller Cm 3+ preferred forming an anion. There were approximately four bound O 2 CMe 1- ligands and one to two inner sphere H 2 O ligands. The conclusion that increasing acetic acid/acetate concentrations increased acetate complexation was corroborated by characterizing (NH 4 ) 2 M(O 2 CMe) 5 (M = Eu 3+ , Am 3+ and Cm 3+ ) using single crystal X-ray diffraction and optical spectroscopy (absorption, emission, excitation, and excited state lifetime measurements)., Competing Interests: The authors have no conflicts of interest with this work., (This journal is © The Royal Society of Chemistry.)