Using secondary ion mass spectrometry (SIMS) we have investigated the concentration vs. depth profile of Nb, thermally diffused into (1 0 0)oriented yttria-stabilized zirconia (YSZ) single crystal substrates. The surface morphology of Nb films and YSZ substrates was analyzed using atomic force microscopy (AFM). The structural disorder and the interface configuration of the samples were investigated by X-ray reflectometry (XRR). Two kinds of substrates were used: as-received (AR) and reduced (R) ones. The R-substrates were obtained by thermal annealing of AR-substrates in air for 2 h at 1250 degrees C. The bulk diffusion coefficients D-T in the temperature range of 780-1000 degrees C, activation energy Q, and the pre-exponential factor, D-0, have been obtained for Nb in YSZ. For the AR single crystals, the results can be well represented by the expression: D(cm(2)s(-1)) = 2.93 x 10(1) exp (-3.91[eV] / k(B)T) The diffusion behaviour of Nb in the R-substrates yields slightly different results. It is concluded that incorporation of Nb into YSZ lattice is governed by the vacancy mechanism. (c) 2006 Elsevier B.V. All rights reserved.