Hans Peter Eugster (November 19, 1925, in Igis, Switzerland – December 17, 1987, in Baltimore, US) was a Swiss-American geochemist, mineralogist, and petrologist.
As a postdoctoral fellow, Eugster studied optical spectroscopy from 1951 to 1952 at the Massachusetts Institute of Technology, where he was also influenced by research on petrology done by James Burleigh Thompson's team at Harvard University.
There, from 1952 to 1958, he studied experimental mineralogy under Hatten Yoder, specializing in high temperatures and aqueous fluid pressures.
In an innovative attempt to extend his studies to a diverse group of chemically more complex natural minerals, Hans devised a buffer technique that allowed laboratory mineralogists for the first time to control fugacity in the redox investigations of phases containing elements of variable valence.
This technical advance opened up a broad range of Fe-bearing phases to quantitative studies, previously unavailable due to lack of compositonal control.