The reaction between N-methydiethanolamine (mdeaH(2)), benzoic acid, FeCl3, and DyCl3 yields a decanuclear coordination cluster, [Dy3Fe7(mu(4)-O)(2)(mu(3)-OH)(2)(mdea)(7)(mu-benzoate)(4)(N-3)(6)]center dot 2H(2)O center dot 7CH(3)OH (1) whose single crystal structure exhibits three and seven crystallographically distinct Dy(III) and Fe(III) ions; six of the Fe(III) ions are pseudo-octahedrally coordinated, whereas the seventh has a trigonal-bipyramidal coordination geometry. Both direct current (dc) and alternating current (ac) magnetic susceptibility studies indicate that, upon cooling, intracluster antiferromagnetic interactions are dominant in 1, yielding a ferrimagnetic spin arrangement. The out-of-phase (chi '') ac susceptibility reveals that 1 undergoes a slow relaxation of its magnetization mainly resulting from the anisotropy of the Dy(III) ions. This slow relaxation has been confirmed both by magnetization measurements on an oriented single crystal of 1 and by the observation of hysteresis loops below 1.9 K. The macroscopic magnetic studies yield an effective energy barrier, U-eff, of 33.4 K for this relaxation, a barrier that is the highest yet reported for a lanthanide(III)-Fe(III) single molecule magnet. The Fe-57 Mossbauer spectra of 1 obtained between 3 and 35 K are consistent with the presence of Fe(III) intracluster antiferromagnetic coupling with slow magnetic relaxation relative to the Larmor precession time, thus confirming, on a microscopic scale, the presence of a barrier to the magnetic relaxation below 35 K. Between 55 and 295 K the Mossbauer spectra reveal paramagnetic behavior with six partially resolved quadrupole doublets, one for the trigonal-bipyramidal Fe(III) site and five for the six pseudo-octahedral Fe(III) sites.