The results of a 13 °C, 9 GHz electron-paramagnetic resonance (EPR) investigation of the sulfate salts of the species formed by the base hydrolyses of solutions containing Mn2+ and Al3+, in the molar concentration range 1≤Mn/(Al+Mn)≤10% is reported. The EPR spectra of the microcrystalline samples consisted of the superposition of two overlapping axial, hyperfine structure (hfs) central transition Mn2+ resonances with g=1.9994(2) D=6.93(3) mT, A=8.56(3) mT, A=8.49(2), P=0.4(1)×10-4 mT, and gn=0.7(1)×10-3, and an unresolved resonance due to exchange coupled Mn2+ ions. The results are consistent with the interpretation that the resolved axial symmetry hfs central transition spectrum arises from Mn2+ ions replacing aluminum in either the tetrahedral or octahedral sites of the tridecameric Keggin-like cation [AlO4Al12(OH)24H2O)12]7+, which is known to form when Al3+ solutions are hydrolyzed under these conditions. The unresolved, exchange coupled spectrum must arise when two or more Mn2+ ions substitute into the same cation. The ratio of the intensities of the axial and exchange coupled Mn2+ centers had a maximum near the stoichiometric molar concentration 7.69% [i.e., Mn/(Al+Mn)=1:13]. The temperature dependence of the EPR spectral intensity and total hfs splitting for the resolved hfs central transition spectrum revealed that a first-order isomorphous phase transition occurred at 120 °C. . [ABSTRACT FROM AUTHOR]