Two new mixed-valence manganese-carboxylate clusters, [(Mn9Mn6IV)-Mn-III(O2CPh)(12)(mu(3)-O)(13)(mu-O)(4)(mu-OMe)(5)(MeOH)(4)( H2O)(5)](2)center dot 1.5PhCO(2)H center dot MeOH center dot 6H(2)O (1, PhCO2H = benzoic acid) and [(Mn9Mn6IV)-Mn-III(O(2)CCh)(12)(mu(3)-O)(13)(mu-O)(4)(mu-OMe)(5)(MeOH)(3 )(H2O)(6)]center dot 0.5MeOH center dot 2.5H(2)O (2, ChCO(2)H= cyclohexanecarboxylic acid) contain new disklike Mn-15 cores. Both 1 and 2 can be synthesized by the conventional manganese redox reaction (MnO4- oxidizing Mn2+) in methanol solution. 2 can be also synthesized via the site-specific ligand substitution reaction from 1. 1 crystallizes in the triclinic space group P (1) over bar, whereas 2 crystallizes in the trigonal space group P (3) over bar. Magnetic study shows that both 1 and 2 have the same ground spin states S-T = 2. Compared to the silence of the out-of-phase ac susceptibility of 1, 2 shows clearly slow magnetic relaxation behavior above 1.8 K due to the dramatically enhanced axial magnetic anisotropy (D = -0.89 and -1.58 cm(-1) for 1 and 2, respectively, which was obtained by fitting the plots of M vs H/T with the program ANISOFIT 2.0).