The title compound, octacalcium copper pentagermanium octadecaoxide, represents a new intermediate phase between CaO and GeO2, and has not previously been reported in the literature. The structure consists of three different Ge sites, two of them on general 8d positions, site symmetry 1, one on special position 4d, site symmetry 2. Three of the five Ca sites occur on 8d positions, site symmtery 1, one Ca is on 4b with site symmetry -1 and one Ca is on 4c with site symmetry 2. All nine O atoms have symmetry 1 (8d position). By sharing common edges, the Ca sites form infinite bands parallel to the c axis, and these bands are interconnected by isolated GeO4 and Ge3O10 units. These (100) layers are stacked along a in an ABAB... sequence, with the B layer being inverted and displaced along b/2.