 In 1983, he was made Assistant Professor (tenured) for American Studies at Johannes Gutenberg University and continued his academic work after completion of his PhD.He interrupted his work at Mainz by taking sabbaticals at Wheaton College (Illinois) (Clyde S. Kilby Professor in 1992) and Guest Professor at Georgia State University in Atlanta (1996 and 1997).In the first year (1996), Siebald produced the monograph "Der verlorene Sohn in der amerikanischen Literatur" - "The Prodigal Son in American Literature" - which was published in 2003.