Our investigations into developing an alkaline based process for spent nuclear fuel (SNF) reprocessing have revealed unique and unexpected solution and solid state species that form in the presence of environmentally relevant ligands (OH-, HCO3-, CO32-, H2O2 and oxalate), the latter two of which are present as radiolysis products. Not only are these species relevant to SNF reprocessing, but they are newly identified complexes that could form under nuclear waste repository conditions and significantly alter the transport properties of the components of SNF. Peroxide exhibits an extraordinary affinity to coordinate with actinides, surprisingly out-competing carbonate and hydroxide ligands for complexation. For instance, H2O2 forms a very stable solution species with U(VI), displacing a carbonate ligand to form the ternary UO2(O2)(CO3)24-. We have thermodynamically characterized this species for the first time, and isolated it as K4[UO2(O2)(CO3)2]•nH2O. At near-neutral pH, uranium forms the previously characterized tri-nuclear complex (UO2)3(CO3)66-. Our studies indicate that peroxide can displace carbonate from this complex, but the structure of the new compound remains un- determined. Plutonium also forms mixed peroxo-carbonate species in solution such as Na8Pu2(O2)2(CO3)6•12H2O, which is the first molecular structure for a dimeric Pu compound. The solubility-limiting species for Ln(III) in carbonate (no peroxide) solution is Ln(CO3)4(H2O)5-, and has been isolated and characterized by single crystal x-ray diffraction for most of the light lanthanides. Spectroscopic evidence indicates that peroxide complexes with the lanthanide carbonates to form a mixed species which is extremely insoluble, in contrast to the mixed uranium peroxo-carbonate species which appears to significantly increase in solubility. Crystallographic data, spectroscopic characterization, and photographs of these and other environmentally relevant lanthanide and actinide complexes identified in near-neutral and alkaline solutions will be presented.