To probe the interface of silicon sol–gel bonded wafers we developed in–situ micromechanical bending test coupled with optical microscopy. The silicon wafers were bonded together at room temperature using sol–gel silica and dried at 60 C and sintered at 600 C. Beam specimens were cut from the bonded wafers, then notched and tested in three-point bending. During bending the crack opening from a notch and the deviation along the interface was observed with an optical microscope. To quantify the interfacial debonding from considering the experimental results, a simple energy balance allows an apparent interfacial fracture surface energy to be determined. Experiments and the determined interfacial surface energies show that the bonding of the silicon wafers depends on the silica sol–gel chemistry and on the temperature of the thermal treatment during the bonding process.