Single crystal structures of lanthanide carbonate and hydroxy-carbonate compounds have been previously reported in the literature, with the majority of these compounds being extended one- to three-dimensional compounds. Very few lanthanide compounds have been isolated that contain molecular moieties, and none have been reported for either erbium or dysprosium. Single crystals of the tetra-carbonate complexes, [C(NH2)(3)](5)[Er(CO3)(4)] center dot 11H(2)O (I) and [C(NH2)(3)](4)[DY(CO3)(4)(H2O)](H3O) center dot 13H(2)O (II), were isolated from concentrated guanidinium carbonate solutions and characterized by single crystal X-ray diffraction studies. Compounds I and II are the first reported molecular carbonate structures for Er and Dy to be characterized via single crystal X-ray diffraction studies. Crystallographic data for I: monoclinic, space group P21/n, a= 8.816.0(6) angstrom, b= 21.0121(14) angstrom, c= 19.6496(14) angstrom, Z = 4. Data for II: tetragonal, space group P4/n, a = b = 15.3199(11) angstrom, c = 7.5129(11) angstrom, Z= 2.