The reactions of [Mn12O12(O2CEt)(16)(H2O)(4)] with phenylphosphinic acid (PhHPO2H) in MeCN and MeCN/CH2Cl2 have led to isolation of [Mn22O12(O2CEt)(22)(O3PPh)(8)(H2O)(8)] (2) and [mn(22)O(12)(O2CEt)(20)(O3PPh)(8)(O2PPh)(2)(H2O)(8)]n (3), respectively, both containing PhPO32- groups from in situ oxidation of PhHPO2-. Complex 2 is molecular and consists of two Mn-9 subunits linked by four additional Mn atoms. Complex 3 contains almost identical Mn-22 units as 2, but they are linked into a one-dimensional chain structure. The Mn22 unit in both compounds is mixedvalence Mn-18(III), Mn-4(II). Solid-state, variable-temperature dc magnetic susceptibility and magnetization measurements were performed on vacuum-dried samples of 2 and 3, indicating dominant antiferromagnetic interactions. A good fit of low-temperature magnetization data for 2 could not be obtained because of problems associated with lowlying excited states, as expected for a high nuclearity complex containing Mn-II atoms. An approximate fit using only data collected in small applied fields indicated an S = 7 or 8 ground state for 2. Solid-state ac susceptibility data established that the true ground state of 2 is S = 7 and that the connected Mn22 units of 3 are ferromagnetically coupled. Both 2 and 3 displayed weak out-of-phase ac signals indicative of slow magnetization relaxation. Singlecrystal magnetization versus applied dc field scans exhibited hysteresis loops for both compounds, establishing them as new single-molecule and single-chain magnets, respectively. Complex 2 also showed steps in its hysteresis loops characteristic of quantum tunneling of magnetization, the highest nuclearity molecule to show such QTM steps. Arrhenius plots constructed from dc magnetization versus time decay plots gave effective barriers to magnetization relaxation (U-eff) of 6 and 11 cm(-1) for 2 and 3, respectively.