Geometry optimization within the framework of density functional theory provides clear evidence of stable fullerene-like cage structures for C40Si20, C36Si24, and C30Si30. In the case of C40Si20, an extensive isomer search shows that the most stable arrangements are those in which the Si atoms and the C atoms form two distinct homogeneous subnetworks. Any other configuration corresponding to spatially separated sets of Si atoms leads to a decrease of the binding energy. Due to charge transfer from Si to C atoms, opposite charges are found in neighboring Si and C sites. Structural stability is ensured via the predominant occurrence of 3-fold bonding for both species.