1-Dimethylaminoxy-2,3,4,5-tetraphenyl-1-silacyclopentadiene (1), 1,2-bis(dimethylaminoxy)-2,3,4,5-tetraphenyl-1-silacyclopentadiene (2) and 1,2-bis(dimethylaminoxy)-2,3,4,5-tetraphenyl-1-germacyclopentadiene (3) have been prepared by the reactions of the corresponding chloro-2,3,4,5-tetraphenyl-1-(sila/germa)cyclopentadienes with LiONMe2. They are yellow crystalline materials, which have been identified by multinuclear NMR spectroscopy (1H, 13C, 15N, 17O, 29Si), mass spectrometry and elemental analysis. The UV–VIS spectra of 1, 2 and 3 show the absorption bands of the silole/germole to be only slightly affected by the Si⋯N interaction, which indicates that orbital interactions of the type lp(N)→σ*(SiC) do not play a significant role and thus the Si⋯N attraction is better interpreted as a dipole interaction. The crystal structures of 2 and 3 reveal planar C4Si and C4Ge rings, with a propeller-like arrangement of the phenyl groups. The SiON and GeON groups contain short Si⋯N distances [2.473(3) and 2.503(3) Å in 2, and 2.535(7) to 2.608(7) Å in 3]. 2 adopts a gauche-gauche conformation for the NOSiON backbone, while in 3, two independent molecules are found in the asymmetric unit, one with an anti-gauche conformation and one with a gauche-gauche conformation.