He is known for his contribution to thermodynamics and molecular physics, in particular, for the discovery of Eucken's law of thermal conductivity, the measurement of the heat capacity of hydrogen at low temperatures, and the development of the Eucken–Polanyi potential theory of adsorption.

In 1905 he began to work in Berlin under Walther Nernst on the energy states of hydrogen and received a doctorate in 1906.

[1] He habilitated in 1911[2] and after the Italo-Turkish War he joined back in 1915 Eucken at the Technische Hochschule Breslau, and from 1930 at the University of Göttingen as a successor of Gustav Tammann.

[5] In 1908, while working with Nernst, Eucken developed a vacuum calorimeter to study heat capacity hydrogen molecules.

His measurements published in 1912, confirmed that the heat capacity of hydrogen drops by a factor of a gas constant R at low temperatures (about 40 K).