A 1973 book co-authored by Ayers, whose radical views during the 1960s and connection to President Obama were widely discussed during the 2008 presidential campaign, is apparently to blame.
Ayers dedicated the book to Sirhan Sirhan, the man who assassinated Robert Kennedy.
Kennedy's son, Christopher, chairs the U. of I. board.
Kennedy criticized the book for celebrating the assassination of his father.
All nine voting trustees either opposed or abstained from the vote because they say they have not seen any signs of remorse from Ayers.
Ayers recently retired as a professor at the University of Illinois at Chicago.