The effect of spillover processes on the activity of a catalyst system consisting of a mixed oxygen ion and electronic conducting support La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-delta) and a metal catalyst (Pt) were investigated. Two types of model single-pellet catalysts were used employing Pt deposited on both sides of a dense LSCF disc pellet. One of these single pellets employed highly disperse, physically non-continuous Pt, in contrast to studies on electrochemical promotion, while the other used a low dispersion continuous film. Driving forces for promoter migration were controlled through the manipulation of the oxygen chemical potential difference across the membrane. Catalyst rate modification was observed in all cases. However, it was found that there is a complex relationship between the rate modification, the driving forces for spillover and the geometrical arrangement of the catalyst on the support (i.e. catalyst dispersion). (C) 2011 Elsevier Inc. All rights reserved.