In this paper the results are presented of work dealing with the microstructure of ZrO2-Ni-Si3N4 diffusion bonds. Joints were made at 1050degreesC with bonding times between 22.5 and 360 minutes. The interfacial structure was studied using optical microscopy, scanning electron microscopy and transmission electron microscopy. The element concentrations in the joint were determined by electron probe microanalysis. It was found that at the ZrO2-Ni interface no reactions take place, whereas at the Si3N4-Ni interface decomposition of Si3N4 occurs, resulting in a solid solution of Si in Ni and porosity due to the recombination of N to N-2.