 He started his career with working at AarhusKarlshamn 1936–1939 before founding his own catalysis company "Haldor Topsøe" in 1940 with which he established himself as engineer in Kongens Lyngby.The company got a production structure in Frederikssund in 1958 and later in Houston, United States.During the 1950s, Topsøe experimented with petition of silicon to semiconductors in the cellar in his home and in 1959, he founded the process Topsil.