A series of isostructural oxalates of Th, U, Np, and Pu have been synthesized. The crystal structure of {C(NH2)(3)}(4)[An(C2O4)(4)]center dot 2H(2)O comprises infinite [An(C2O4)(4)](4n-) chains, guanidinium cations and water molecules. Each An atom is connected to five oxalate anions, two of which act as bridging tetradentate ligands, while the other three function as terminating bidentate ligands. Electron and IR spectra of the compounds as well as the results of thermal analysis correlate with their structure. An actinide contraction effect is demonstrated by a decrease in the An-O interatomic distances and the volumes of An atom coordination polyhedra and Voronoi-Dirichlet polyhedra. Ten-coordinate An oxalates are discussed in terms of continuous shape measures approach.