Ultraviolet–visible spectroscopy is one of the oldest and quickest methods of studying the binding activity of various molecules. The absorption of UV-light takes place at a time-scale of picoseconds, hence the individual signals from the species can be observed. At the same time, the intensity of absorption directly correlates with the concentration of the species, which enables easy calculation of the association constant. Most commonly, either the host or the guest is transparent to UV-light, whilst the other molecule is UV-sensitive. The change in the concentration of the UV-sensitive molecules is thus monitored and fitted onto a straight line using the Benesi–Hildebrand method, from which the association constant can be directly calculated.