The first two families of polyoxometalate-based single-molecule magnets (SMMs) are reported here. Compounds of the general formula [Ln(W5O18)(2)](9-) (Ln(III) = Tb, Dy, Ho, and Er) and [Ln(SiW11O39)(2)](13-) (Ln(III) = Tb, Dy, Ho, Er, Tm, and Yb) have been magnetically characterized with static and dynamic measurements. Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for [Ln(W5O18)(2)](9-) (Ln(III) = Ho and Er) and [Ln(SiW11O39)(2)](13-) (Ln(III) = Dy, Ho, Er, and Yb). Among them, only the [Er(W5O18)(2)](9-) derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligand-field parameters and the splitting of the J ground-state multiplet of the lanthanide ions have been also estimated.