First-principles studies of metal/alpha-Al2O3 interfaces have revealed strong interfacial stoichiometry effects on adhesion. The metals included Al, Ni, Cu, Au, Ag, Rh, Ir, Pd, Pt, Nb, and beta-NiAl. Metallic and ionic-covalent adhesive bonding effects were found in varying amounts depending on whether the interfacial stoichiometry is stoichiometric, oxygen-rich, or aluminum-rich in a qualitative way. A semiempirical but physically sensible understanding ensues for the effects of interfacial stoichiometry and reveals the underlying strong correlation of the interfacial adhesion with the physical properties of the bulk materials that join and form the interface. The metallic component of the bonding was found to be related to the ratio of (B/V)(1/2), where B and V are the bulk modulus and molar volume of the metal, respectively. In like manner, the ionic-covalent component of the bonding could be related to the enthalpy of oxide formation of the bulk metal. A unified model is proposed to describe the adhesion of metal/alumina interfaces with interfacial stoichiometry effects, and the model is also expected to be valid for other metal-oxide interfaces.