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)(24)(H2O)(12)](SO4)(3.55). These crystals have been studied using Raman microscopy at 300 and 77 K and compared to 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(2) and nu(4)SO(4)(2-) triplets at 446, 459 and 496 cm(-1) and 572, 614 and 630 cm(-1). The nu(1) is observed as a single band at 990 cm(-1), partly overlapped by the nu(3) triplet at 979, 1009 and 1053 cm(-1) of the sulfate group in the Al-13 sulfate structure. Furthermore the band at 726 cm(-1) is assigned to an Al-O mode of the polymerised Al-O-Al bonds in the Al-13 Keggin structure. For the first time the OH-stretching region of the basic aluminum sulfate has been reported. The 77 K spectrum shows the presence of 3 crystal water bands at 3035, 3138 and 3256 cm(-1) accompanied by 3 Al- H2O bands at 3354, 3418 and 3498 cm(-1) and 4 Al-OH bands at 3533, 3584, 3671 and 3697 cm(-1).