Some chemical vapour deposition polycrystalline diamond layers are deposited using a gas mixture (CH4 + D-2) and a deuterium concentration of 3 x 10(19) cm(-3), originating from the vector gas, is found in these as-grown samples. Samples first annealed at 1200 degreesC, are exposed to a deuterium microwave or RE plasma, and the deuterium diffusion profiles are analysed. These profiles are explained in term of trapping on plasma induced defects near the surface and trapping on inter- and intragranular defects in the bulk. The mean free paths of deuterium are calculated by fitting the deuterium diffusion profiles and compared to the grain sizes. (C) 2002 Elsevier Science B.V. All rights reserved.