Then he obtained a research grant from the University of Cologne and worked with Reinhold Merkelbach. He was habilitated to university teaching in 1969 and became Professor in Classical Philology in 1970. He retired in 1998 but continued working with the university.
Van Thiel graduated defending a thesis on the Greek Alexander Romance, and published monographs on Petronius, the lost Greek novel of Lucius (a major source of The Golden Ass, by Apuleius), and the Alexander Romance itself.
Starting from the 1980s, he researched the Homeric poems and published critical editions of both, including two online editions of the D scholia to the Iliad. Van Thiel used a neo-analytical approach, and suggested that both the Iliad and the Odyssey are made of shorter poems which underwent an editing process in several stages, which merged them into the epics that we have today. Accordingly, his books on the subjects were titled Iliaden und Iliad [Iliads and Iliad] and Odysseen [Odysseys]. In 2009, he published German translations of both the Homeric poems.
He also published handbooks of Greek and Latin palaeography, with Merkelbach.
Van Thiel, H. (1971a). Der Eselsroman. Zetemata, 54-I. Vol.I: Untersuchungen. München: C. H. Beck.
Van Thiel, H. (1971b). Petron. Überlieferung und Rekonstruktion. «Mnemosyne», Supplement 20. Leiden: E. J. Brill.
Van Thiel, H. (1972a). Der Eselsroman. Zetemata, 54-II. Vol.II: Synoptische Ausgabe. München: C. H. Beck.
Van Thiel, H. (1972b). Mittellateinische Texte. Ein Handschriften-Lesebuch. Studienhefte zur Altertumswissenschaft, 14. Göttingen: Vandenhoeck & Ruprecht.
Van Thiel, H., ed. (1972c). Abenteuer eines Esels oder die Verwandlung des Lukios. Der griechische Eselsroman. Sammlung Tusculum. München: Heimeran Verlag.
Van Thiel, H., ed. (1974). Leben und Taten Alexanders von Makedonien. Der griechische Alexanderroman nach der Handschrift L. Texte zur Forschung, 13. Darmstadt: Wissenschaftliche Buchgesellschaft.
Van Thiel, H. (1982). Iliaden und Ilias. Basel • Stuttgart: Schwabe & Co.
Van Thiel, H. (1988). Odysseen. Basel • Stuttgart: Schwabe & Co.
Homerus (1991). van Thiel, H. (ed.). Odyssea. Bibliotheca Weidmanniana, 1. Hildesheim • Zürich • New York: Weidmann.
Homerus (1996). van Thiel, H. (ed.). Ilias. Bibliotheca Weidmanniana, 2. Hildesheim • Zürich • New York: Weidmann.
Van Thiel, H., ed. (2000a). Homerpur. Köln: Universitäts- und Stadtbibliothek.
Universität zu Köln (19 October 2014). "Prof. Dr. Helmut van Thiel". Universität zu Köln | Philosophische Fakultät | Altertumskunde. Archived from the original on 25 October 2022. Retrieved 6 October 2025.