During its heyday IG Farben was the fourth-largest company in the world, after General Motors, U.S. Steel and Standard Oil.

IG Farben was founded on December 25, 1925 as a merger of the following six companies: BASF (27.4 percent of equity capital), Bayer (27.4 percent), Hoechst including Cassella and Chemische Fabrik Kalle (27.4 percent), Agfa (9.0 percent), Chemische Fabrik Griesheim-Elektron (6.9 percent) and Chemische Fabrik vorm. Weiler Ter Meer (1.9 percent). In 1926 IG Farben had a market capitalization of 1.4 billion Reichsmark and a workforce of 100,000 people, of which 2.6 percent were university educated, 18.2 percent were salaried professionals and 79.2 percent were workers.

The IG Farben Building, headquarters for the conglomerate in Frankfurt am Main, Germany, was completed in 1931.

In 1938 the company had 218,000 employees.

In 1941, an investigation exposed a "marriage" cartel between John D. Rockefeller's United States-based Standard Oil Co. and I.G. Farben.

It also brought new evidence concerning complex price and marketing agreements between DuPont, a major investor in and producer of leaded gasoline, U.S. Industrial Alcohol Co. and their subsidiary, Cuba Distilling Co. The investigation was eventually dropped, like dozens of others in many different kinds of industries, due to the need to enlist industry support in the war effort. However, the top directors of many oil companies agreed to resign and oil industry stocks in molasses companies were sold off as part of a compromise worked out.

At its peak in 1944, this factory made use of 83,000 slave laborers.^[22] The pesticide Zyklon B, for which IG Farben held the patent, was manufactured by Degesch (Deutsche Gesellschaft für Schädlingsbekämpfung), which IG Farben owned 42.2 percent of (in shares) and which had IG Farben managers in its Managing Committee.

Of the 24 directors of IG Farben indicted in the so-called IG Farben Trial (1947-1948) before a U.S. military tribunal at the subsequent Nuremberg Trials, 13 were sentenced to prison terms between one and eight years. Some of those indicted in the trial were subsequently made leaders of the post-war companies that split off from IG Farben, including those who were sentenced at Nuremberg.

Today only Agfa, BASF, and Bayer remain, Hoechst having in 1999 demerged its industrial chemical operations to Celanese AG and merged its life-sciences businesses with Rhône-Poulenc's to form Aventis.