Reaction of a solution containing a tetravalent actinide An(IV), a trivalent actinide An(III) and a single-charged cation such as hydrazinium in presence of oxalic acid in acidic medium under controlled conditions, leads to the precipitation of mixed An(IV)-An(III) oxalate compounds never reported before. By varying the (An$^{IV}$,An$^{III}$) pair and depending on the An$^{IV}$-An$^{III}$ ratio, two original series were obtained (N$_2$H$_5$,H$_3$O)$_{2+x}$An$^{IV}$$_{2-x}$An${III}$$_x$(C$_2$O$_4$)$_5$.4H$_2$O (1) and (N$_2$H$_5$,H$_3$O)$_{1-x}$(An$^{III}$$_{1-x}$An$^{IV}$$_x$(C$_2$O$_4$)$_2$.H$_2$O).4H$_2$O (2). The crystal structures were identified from powder diffraction patterns by analogy to hydrazinium uranium (IV) lanthanide (III) oxalates whose structures were solved recently by single-crystal X-ray diffraction. Complementary investigations by UV-visible and Infra-Red spectroscopies and thermogravimetric analysis confirm the presence of both tetravalent and trivalent states of actinides in structures (1) and (2) and the role of single-charged cation and water molecules.The originality of both structures is the existence of a mixed crystallographic site for the tetravalent actinide and the trivalent one, the charge balance being ensured by the adjustment of the single-charged ions within the structure. The main difference is that actinides are ten-coordinated in (1) and nine-coordinated in (2).This is the first evidence of a mixed actinide(IV)-actinide(III) site in an oxalate structure.