After Velvet Revolution in 1989, Sion was appointed associate professor in the painting studio at the Academy of Fine Arts. He took part in a symposium on Art Informel (1991) and was featured at the exhibition "Czech Art Informel: Pioneers of Abstraction 1957–1964". In 1993 he had retrospective exhibitions in Roudnice nad Labem, The Václav Špála Gallery in Prague and the Ostrava House of Art. In 1996 Sion had another retrospective at the Rudolfinum Gallery in Prague.