Tuesday night's debate will be broadcast radio-only at 7 p.m. by the Illinois Radio Network, and on stations in Chicago and Bloomington.
The second debate at the studios of WLS-TV in Chicago will be televised Oct. 21. The third debate Oct. 26 will be broadcast by public television stations.
Obama, 43, rose to national attention when he delivered the keynote address at the Democratic National Convention in Boston in July. The winner of the Nov. 2 election will become the nation's only African-American senator -- only the fifth black senator in history.
A recent St. Louis Post-Dispatch poll showed Obama, an Illinois state senator, ahead of Keyes, a Maryland talk show host recruited as a replacement candidate, by 45 points, with just a 3.5-point margin of error.