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.