Two new heterometallic cubane molecules have been synthesized, High-frequency electron paramagnetic resonance and magnetization measurements indicate that [Mn3Ni(hMP)(3)O(N-3)(3)(C7H5O2)(3)] (1) displays a well-isolated S = 5 ground state (Delta E > 120 K), with g 2.0, D = -0.23 cm(-1), and ferromagnetic Mn-Mn exchange Interactions competing with antiferromagnetic Ni-Mn interactions.[Mn3Zn(hmp)(3)O(N-3)(3)(C3H5O2)(3)] (2) possesses a S = 6 ground state (Delta E > 105 K), with g = 2.0, D = -0.14 cm(-1), and ferromagnetic Mn-Mn exchange interactions. Magnetization vs magnetic field data for oriented single crystals of 1 and 2 indicate that both complexes are single-molecule magnets.