The solution‐state interactions of plutonium and berkelium with the octadentate chelator 3,4,3‐LI(1,2‐HOPO) (343‐HOPO) were investigated and characterized by X‐ray absorption spectroscopy, which revealed in situ reductive decomposition of the tetravalent species of both actinide metals to yield Pu(III) and Bk(III) coordination complexes. X‐ray absorption near‐edge structure (XANES) measurements were the first indication of in situ synchrotron redox chemistry as the Pu threshold and white‐line position energies for Pu‐343‐HOPO were in good agreement with known diagnostic Pu(III) species, whereas Bk‐343‐HOPO results were found to mirror the XANES behavior of Bk(III)‐DTPA. Extended X‐ray absorption fine structure results revealed An—OHOPO bond distances of 2.498 (5) and 2.415 (2) Å for Pu and Bk, respectively, which match well with bond distances obtained for trivalent actinides and 343‐HOPO via density functional theory calculations. Pu(III)‐ and Bk(III)‐343‐HOPO data also provide initial insight into actinide periodicity as they can be compared with previous results with Am(III)‐, Cm(III)‐, Cf(III)‐, and Es(III)‐343‐HOPO, which indicate there is likely an increase in 5f covalency and heterogeneity across the actinide series. X‐ray absorption spectroscopy was used to probe the interactions between an octadentate hydroxypyridinone chelator and two transuranic elements in microgram quantities – plutonium and berkelium – within buffered solutions. Despite the precedence for chelation‐driven stabilization of the tetravalent oxidation state of actinides with this ligand, in situ reductive decomposition yielded plutonium(III) and berkelium(III) coordination complexes.