Low temperature bonding of Si wafers has been studied utilizing RIE-mode plasma activation. The hydrophilic Si and thermally oxidized Si wafers were exposed to N2, Ar or O2 plasma prior to bonding in air or vacuum. After plasma treatment the wafers were cleaned in RCA-1 solution and/or DI-water. Strong bonding was achieved at 200°C with all the investigated plasma gases if proper bonding and cleaning procedure is used. Extended RCA-1 cleaning deteriorated the bond strength but a short cleaning improved bonding. We found out that the activation of the thermal oxide has a larger influence on the bond strength than the activation of the native oxide surface in Si/oxide wafer pairs. We suggest that the plasma treatment induces a highly disordered surface structure which enhances the diffusion of the water from the bonded interface. As a result of the plasma exposure the number of the surface OH-groups is highly increased enabling strong bonding at a low temperature.