Quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets (SMM's). Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)(16)(H2O)(4)] . solvate. The two Mn-12 complexes are isomers. The magnetization versus magnetic field hysteresis loops are quite different for the two isomeric Mn-12 complexes. One Mn-12 Complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn-12 isomer. The unusual orientation of Jahn-Teller elongation axes in one isomer leads to greater rhombic zero-field splitting that is the origin of the faster magnetization tunneling. In addition to data for Mn-12 complexes, magnetization relaxation rate versus temperature responses of three mixed-valence Mn-4 complexes are also reported. In all three cases the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-independent region as well as a temperature-dependent region. The temperature-independent relaxation rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.