The short-term changes in the electrical conductivity of Ni-YSZ composites (cermets) suitable for use in Solid Oxide Fuel Cells (SOFC) were measured by an in-situ 4-point DC technique. The isothermal reduction was carried out in dry, humidified or wet hydrogen at temperatures from 600 to 1000 C. While the cermets reduced at 600 degrees C showed a stable conductivity of 1000-1200 S/cm, rapid initial conductivity loss was observed at elevated temperatures. At 1000 degrees C the conductivity degraded nearly instantaneously to about 800 S/cm, and continued to decline fast to about 400 S/cm. At 850 degrees C. the presence of steam did have an accelerating effect on the conductivity loss. Scanning Electron Microscopy of cermets reduced in different conditions showed increasing particle size and loss of metal-to-metal percolation in the samples reduced at higher temperatures. The short-term changes in conductivity were modelled using two different semi-empirical approaches. Thermodynamic calculations were carried out to assess the vaporisation of Ni in the conditions tested. The rate and mechanisms of conductivity degradation due to Ni particle growth are discussed in light of the measurements, modelling and literature data. (C) 2011 Elsevier B.V. All rights reserved.