German physicist who was corecipient, with R.G.W. Norrish and George Porter, of the 1967 Nobel Prize for Chemistry for work on extremely rapid chemical reactions.

Eigen was educated in physics and chemistry at the University of Göttingen (Ph.D., 1951). He worked at the university's Institute of Physical Chemistry from 1951 to 1953, when he joined the Max Planck Institute for Biophysical Chemistry, Göttingen, where he became director in 1964 and later chairman.

Eigen was able to study many extremely fast chemical reactions by a variety of methods that he introduced and which are called relaxation techniques. (relaxation phenomenon) These involve the application of bursts of energy to a solution that briefly destroy its equilibrium before a new equilibrium is reached. Eigen studied what happened to the solution in the extremely brief interval between the two equilibria by means of absorption spectroscopy. Among specific topics thus investigated were the rate of hydrogen ion formation through dissociation in water, diffusion-controlled protolytic reactions, and the kinetics of keto-enol tautomerism.