The binding energy of an electron in a material is a fundamental characteristic, which determines a wealth of important chemical and physical properties. For metal organic frameworks this quantity is hitherto unknown. We present a general approach for determining the vacuum level of porous metal organic frameworks and apply it to obtain the first ionization energy for six prototype materials including zeolitic, covalent, and ionic frameworks. This approach for valence band alignment can explain observations relating to the electrochemical, optical, and electrical properties of porous frameworks.