U.S. chemist who interpreted the behaviour and structure of organic compounds in the light of valence theory and applied the methods of physical chemistry to organic chemistry.

Stieglitz received his Ph.D. from the University of Berlin (1889) and later was associated with the University of Chicago, where he headed the chemistry department (1915–33). He studied molecular rearrangements, catalysis, the theory of chemical indicators, and the structure of organic nitrogen compounds. Stieglitz introduced theories of ionization and chemical equilibrium in Elements of Quantitative Chemical Analysis, 2 vol. (1911–12).