Two heterometallic Co(III)-Gd(III) nanomagnets (Co2Gd6 and Co2Gd9) with defective dicubane-like cores were isolated from the same set of reactants by varying the reaction conditions. These are the first examples of cobalt(III)-gadolinium(III) phosphonate compounds and a rare class of compounds with large 4f ratio among the reported 3d-4f complexes. Magnetic studies reveal large magnetic entropy changes for both complexes (-ΔSm = 27.81 and 33.07 J kg(-1) K(-1), respectively at 3 K and 7 T).