In an integrated fuel cycle such as considered for generation IV nuclear systems, actinides co-conversion processes play an important role by producing mixed actinides compounds used as starting materials for fuel re-fabrication. Among the different co-conversion routes considered, oxalic co-precipitation is one of the investigated ways to synthesised actinides mixed oxides. Oxalic co-conversion of a solution containing a tetravalent actinide An(IV) and a trivalent actinide An(III) under controlled conditions leads to the precipitation of two original series of mixed An(IV)–An(III) oxalate compounds (An(IV)=Th, Np, U or Pu and An(III)=Pu or Am) characterized by an unexpected An(IV)–An(III) crystallographic mixed site which induces a local homogeneity of the composition at a molecular scale. These families of mixed oxalates correspond to two solid solutions of type M 2+x An IV 2−x An III x (C 2 O 4 ) 5 ·nH 2 O and M 1−x [An III 1−x An IV x (C 2 O 4 ) 2 ·H 2 O]·nH 2 O (M=single charged cation), with hexagonal or tetragonal symmetry, respectively.