Double and triple thorium oxalates have been synthesized by successive evaporation of a solution of thorium nitrate, ammonium, or hydrazinium oxalate and hydrogen peroxide. Compounds M6Ca2Th2(C2O4)9·nH2O (M = NH4+, N2H5+) (1) were first accidentally obtained due to the unexpected presence of calcium as impurities in the thorium nitrate used. Further evaporation led to the M2Th2(C2O4)5·nH2O (M = NH4+, N2H5+) (2) precipitation. After an additional evaporation of 2 weeks, compounds (2) transform into M4Th(C2O4)4·nH2O (M = NH4+, N2H5+) (3). In the three compounds, thorium is coordinated by five bidentate oxalate ions. In (1) Th(C2O4)5 entities share an oxalate ion to form dimeric units further connected by Ca(C2O4)5 (M = NH4+) or Ca(C2O4)5(H2O) (M = N2H5+) entities to build three-dimensional (3D) frameworks. Compounds (2) exhibit a 3D honeycomb thorium oxalate framework. Compounds (3) contain chains of oxalate-shared Th(C2O4)5 entities, whereas, in solution, the compound (NH4)2Th2(C2O4)5·1.5H2O (2) reacts with oxalate ions and turns into compound (NH4)4Th(C2O4)4·4H2O (3); in the solid state the chains of the compound (3) condense to form the 3D framework of the compound (2) and release solid ammonium oxalate.