The co-firing characteristics of ZnO varistor and NiZn ferrite thick films were investigated, the objective being to create an integrated passive device. Bismuth oxide additions were used to increase the shrinkage of the ferrite during sintering. Dilatometry analysis proved that well-matched shrinkage characteristics could be obtained for bismuth oxide-modified ferrite compositions and the starting varistor material. Cross-boundary diffusion across the co-fired varistor-ferrite interface occurred during sintering, but application of a barrier layer of pure zinc oxide reduced this phenomenon considerably. By tailoring the shrinkage, strong, crack-free co-fired layers with no evidence of cracking or delamination were obtained. No deleterious effects were observed due to electroding of the varistor and ferrite materials. (C) 2008 Elsevier Ltd. All rights reserved.