Diffusion-bonded surfaces inside a cavity usually have little effect on a laser beam when oriented perpendicular to the beam direction. They may have a severe effect on lasing efficiency and mode structure when oriented parallel to the beam. Characterization of the interface between diffusion bonded elements and the understanding of its effect on lasing is, Therefore, important. Isolating the effect of the bonding interface from effects such as an index difference between the bulks, we used bonded BK7 slabs as the bonded element. We measured the reflections arising from the bonding interface, and then put the slabs as a passive element inside a hemispherical resonator. It was found that even for similar materials, with no refraction index difference, significant reflections occur at the bonding interface for oblique angles of incidence. When put intracavity parallel to beam axis the bonded slabs caused a significant power loss and forced higher order lasing modes. The power loss depended on both, the transmission through the interface and the size of the interface cross section relative to beam diameter.