Kenneth Sanborn Pitzer was an American physical and theoretical chemist, educator, and university president. He was described as "one of the most in?uential physical chemists of his era" whose work "spanned almost all of the important ?elds of physical chemistry: thermodynamics, statistical mechanics, molecular structure, quantum mechanics, spectroscopy, chemical bonding, relativistic chemical effects, properties of concentrated aqueous salt solutions, kinetics, and conformational analysis." Kenneth S. Pitzer was born in Pomona, California, in 1914. He received his B.S. in 1935 from Caltech and his Ph.D. in chemistry from UC Berkeley just two years later. He was immediately appointed to the faculty of the Department of Chemistry at Berkeley, where he spent most of his distinguished career. As a scientist he was known for his work on the thermodynamic properties of molecules. While still a graduate student he discovered that hydrocarbon molecules do not rotate unhindered around their C-C bonds. There is in fact a barrier to internal rotation, an important discovery upsetting the conventional wisdom and affecting the thermodynamic properties of hydrocarbons. Some of his work is summed up in the Pitzer equations describing the behavior of ions dissolved in water. During his long career he won many awards, most notably the National Medal of Science and the Priestley Medal.