Executive Order 12343: Designation of Certain Officers to Act as Secretary of State [open pdf - 90KB]
Alternate Title: EO 12343: Designation of Certain Officers to Act as Secretary of State
"During any period when, by reason of absence, disability, or vacancy in office, neither the Secretary of State nor the Deputy Secretary of State, is available to exercise the powers or perform the duties of the Office of the Secretary, an officer from the Department of State who has been appointed by the President, by and with the advice and consent of the Senate, in such order as the Secretary of State may from time to time prescribe, shall act as Secretary. If no such order of succession is in effect at that time, then such officers shall act as Secretary in descending order of rank, as established by the listing of their offices in Sections 5314 or 5315 of Title 5 of the United States Code, and at each level of the Executive Schedule in the order in which they shall have taken the oath as such officers."
Executive Order 12343; EO 12343
National Archives: http://www.archives.gov/