Relativistic multireference CASSCF/CASPT2 calculations have been performed on the mixed caesium actinide oxide molecules Cs(2)AnO(4) with An = U, Np, Pu and Am. Probing the lowest-energy spin multiplicities the spin-orbit-free (SF) ground and low-energy excited states have been evaluated and characterised. After optimizing the molecular geometries of the SF ground states further calculations have been performed taking into account spin-orbit (SO) coupling. The SO ground and vertical low-lying excited states have been characterised. (C) 2018 The Author. Published by Elsevier B.V.