A range of per-deuterated cyclic poly(dimethylsiloxanes) (PDMS) have been conveniently prepared in a high dilution reaction from bis(trideuteriomethyl)diphenylsilane using a catalytic amount of trifluoromethanesulphonic (triflic) acid in the presence of water as proposed. The cyclic and linear per-deuterated polymers produced have been separated and the cyclics fractionated by preparative gel permeation chromatography (GPC). The resulting sharp fractions of per-deuterated cyclic PDMS have been obtained in up to 0.6 gram quantities ranging in size from 45 to 605 number-average number of skeletal bonds, ail with dispersities less than 1.2. The purity of the samples obtained has been verified by various analytical techniques and these materials will allow the first studies of the conformations and properties of cyclic polymers in chemically identical blends using neutron scattering.