Adoula attempted negotiation talks with Moise Tshombe, president of the secessionist Katanga province, but failed to reach any meaningful agreement. UN Secretary-General U Thant proposed a plan to end the secession of Katanga, but Tshombe made no moves to implement it. In response, Adoula demanded that the UN intervene, and place the plan into effect, by force if necessary — Tshombe surrendered on January 15, 1963.
In order to solidify his moderate stance of leadership, in July 1963 Adoula removed his cabinet ministers with the most extreme views, attempting to create a group with a balance of Katanga and Lumumbist members, and a few months later submitted a new federal constitution to the parliament. However, various rebellions continued, and Adoula resigned in 1964 to be replaced by Tshombe. He was then the ambassador to the United States and Belgium, and then became foreign minister from 1969 until 1970, when he retired from politics. He died in Lausanne, Switzerland.