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-methyl-piperidine (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
.