When the polycyclic alumosiloxane (Ph 2 SiO) 8 [AlO(OH)] 4 , which may be isolated as the diethyl ether adduct (Ph 2 SiO) 8 [AlO(OH)] 4 ·4OEt 2 , is allowed to react with the double N-methylpiperidine (nmp) adduct of monochloroalane, AlH 2 Cl·2nmp (1) (crystal structure analysis), the polycycle (Ph 2 SiO) 8 [AlO(O) 0.5 ] 4 ·2nmp (2) is obtained. Compared to the starting material and apart from the coordinating bases, the compound formally has lost two water molecules. The structure of (Ph 2 SiO) 8 [AlO(O) 0.5 ] 4 ·2nmp (2) can be derived from (Ph 2 SiO) 8 [AlO(OH)] 4 by substituting the central Al 4 (OH) 4 motif through an Al 4 O 2 entity which consists of a central Al 2 O 2 ring coordinated to two further aluminum atoms through almost trigonal planar oxygen atoms. Using tris(ethylene)diamine (ted) as base and reacting it with (Ph 2 SiO) 8 [Al(OH)] 4 , we have been able to isolate and completely characterize an intermediate on the way to these formally condensed alumosiloxane polycycles like in (Ph 2 SiO) 8 [AlO(O) 0.5 ] 4 ·2nmp (2). It has the composition (Ph 2 SiO) 8 [AlO(O) 0.25 ] 4 ·(OH·ted) 2 ·(OH 2 ·ted) (3) and has, compared to the starting material, the same number of hydrogen, oxygen, aluminum and silicon atoms within the inner molecular framework. Nevertheless, its structure is very different: whereas half of the molecule is structurally similar to (Ph 2 SiO) 8 [AlO(OH)] 4 , with OH-groups forming hydrogen bridges to the nitrogen atoms of ted and connecting two aluminum atoms, the other half contains a unique oxygen atom which is in an almost planar trigonal bonding mode to three aluminum atoms. Furthermore, this part of the molecule has an aluminum atom to which a water molecule is coordinated, one of the hydrogen atoms being involved in hydrogen bonding to a further tris(ethylene)diamine (ted). This structure gives some important insights in the possible mechanism of the “condensation reaction” within (Ph 2 SiO) 8 [AlO(OH)] 4 .