The tridecameric aluminum polymer [AlO4Al12(OH)(24)(H2O)(12)](7+) was prepared by forced hydrolysis of Al3+ up to an OH/Al molar ratio of 2.2. Upon addition of sulfate the tridecamer crystallised as the monoclinic basic aluminum sulfate Na-0.1[AlO4Al12(OH)(1)2(H2O)(12)](SO4)(3.55). These crystals have been studied using FT-Raman microscopy and compared to the basic aluminum nitrate, Na2SO4. xH(2)O and Al-2(SO4)(3). xH(2)O. The Raman spectrum of basic aluminum sulfate is dominated by two broad bands which are assigned to the nu(1) and nu(3) bands at 981 and 1051 cm(-1) of the sulfate group in the Al-13 sulfate structure. Furthermore the band at 724 cm(-1) is assigned to an Al-O mode of the polymerised Al-O-Al bonds in the Al-13 Keggin structure. The sharp band at 1066 cm(-1) and the minor band at 1384 cm(-1) are interpreted to be due to a small amount of nitrate impurity on a different position in the structure than the nitrate present in the Al-13 nitrate crystal structure, based on the shift in band position of both the nu(1) symmetric stretching and nu(3) asymmetric stretching modes.