Hydrophilic silicon surfaces become hydrophobic without microroughening after 200°C low energy hydrogen plasma cleaning. The fully hydrogen-terminated silicon surfaces do not bond to each other, not even by the application of external pressure. A subsequent 400 to 600°C, 4 min thermal treatment in ultrahigh vacuum converts the wafer surfaces to hydrophilic and bondable which can be attributed to desorption of hydrogen from the surfaces. Hydrophobic silicon surfaces prepared by a dip in HF (without subsequent water rinse) are terminated by H and a small amount of F, or by H and a small amount of OH (after subsequent water rinse). Hydrogen bonding of Si-F⋯(HF)⋯H-Si or Si-OH⋯(HOH)⋯OH-Si across the two mating surfaces appears to be responsible for room temperature spontaneous hydrophobic or hydrophilic wafer bonding, respectively.