A series of small cyclic per-deuterated dimethylsiloxanes have been prepared in good yield via a new convenient synthetic route. The cyclic per-deuterated dimethylsiloxanes produced have been characterized by gas liquid chromatography (GLC), gel permeation chromatography (g.p.c.), infra-red absorption spectroscopy (i.r.) H-1, C-13, and Si-29 nuclear magnetic resonance spectroscopy (n.m.r.), gas chromatography/electron ionization mass spectrometry (GC/EI-MS) and also by gas chromatography/ammonia chemical ionization mass spectroscopy (GC/CI-MS). The results of the various techniques showed the main products to be the per-deuterated cyclic tetramer, pentamer and hexamer-with small amounts of higher cyclics also present. GC/EI-MS and GC/CI-MS spectra were obtained for all of the cyclic species from the tetramer to the nonamer inclusive. No evidence was found for the production of linear per-deuterated PDMS via this route. A sample of the per-deuterated cyclic oligomers was polymerized to a high molecular weight per-deuterated linear polymer and analysed by g.p.c.