Microstructural characterization of a high‐Al2O3substrate containing cofired thick‐film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al2O3grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO2and reduced rutile (TiO2‐x). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al2O3grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl2Si2O8), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α‐cristobalite (SiO2), and α‐cordierite (Mg2Al4Si5O18). In addition, small rutile particles precipitated within the