X-ray photoelectron spectroscopy (XPS) is a non-destructive method for investigating the chemical composition of surfaces (a few nanometres) of solid materials such as massive solids, powders and thin films. The examination method can be used both qualitatively and quantitatively. In addition to a purely chemical composition, chemical oxidation states of chemical elements can also be determined in many cases. The detection limit, depending on the element, is approx. 0.1-1%.

The method is based on the analysis of the kinetic energy of photoelectrons, i.e. electrons that are excited and emitted by X-rays. The element-specific binding energies are the basis for spectroscopy. The detection of photoelectrons enables the high surface sensitivity of the method, in contrast to other techniques such as EDX (energy dispersive X-ray spectroscopy), which is typically used with electron microscopes.