The trend for integrating and miniaturizing electronic devices requires the co-firing of ferrites and Ag electrodes at 900°C. Here we report on compatibility studies using Z- and Y-type BaCo hexaferrites, which are interesting for microwave applications. Samples with compositions BaB3BCoB1.4BCuB0.6BFeB24BOB41 Band BaB2BCoB0.6BCuB1.4BFeB12BOB22 Bwere prepared with solid-state reactions at 1250°C and 1000°C, respectively. For compatibility tests, samples with additions of 0-25 wt.% of Ag were sintered at 900°C for 3h. The ceramics were characterized with X-ray powder diffraction and electron microscopy combined with energy-dispersive analysis. When co-sintered with Ag the hexaferrites decomposed to BaFeB12BOB19B and (Co,Cu)FeB2BOB4B or BaFeB2BOB4B. The incorporation of Ag in the spinel phase was also detected. The degree of decomposition increased with the amount of Ag and finally resulted in the complete disintegration of the samples with minimum 23 wt.% of Ag. The influence of the observed phase-composition changes on the electromagnetic properties and their dependence on frequency was evaluated.