In 1951, he became a full professor at his alma mater; at 34 years old, he was the youngest ever full professor at the science faculty in Brussels. In 1959, he was appointed director of the International Solvay Institute in Brussels, Belgium. In that year, he also started teaching at the University of Texas at Austin in the United States, where he later was appointed Regental Professor and Ashbel Smith Professor of Physics and Chemical Engineering. From 1961 until 1966 he was affiliated with the Enrico Fermi Institute at the University of Chicago and was a visiting professor at Northwestern University. In Austin, in 1967, he co-founded the Center for Thermodynamics and Statistical Mechanics, now the Center for Complex Quantum Systems. In that year, he also returned to Belgium, where he became director of the Center for Statistical Mechanics and Thermodynamics.