Resolution 334 B (XI) of the UN Economic and Social Council was adopted which established cooperation between the United Nations and the UIA, and confirmed at ECOSOC's 16th Session. "The Committee unanimously expressed its appreciation of the value and usefulness of the Yearbook of International Organizations published by the Union of International Associations.

Following the war and the dispersion of its archives, the UIA resumed activity as an institute, separate from the Mundaneum.

In 1949, representatives of the UIA, the United Nations, and the Interim Committee of Non-Governmental Organizations met and decided not to attempt to rebuild the UIA as a federation of international associations, but to retain its title and its programme as a centre for documentation, study, service, and the promotion of closer relations between international associations.

The loss to Otlet was enormous, for even though in their later years they worked very little together, they kept constantly in touch, speaking for hours together on the telephone. A year later on 10 December 1944, Otlet died.

Henri La Fontaine died on 14 May 1943, leaving his fortune and his library in two equal shares to the UIA and the International Peace Bureau, thus giving an ultimate proof of his faith in the ideals for which he had striven throughout his entire life.

The work of the UIA was reduced to documentary activities and the publication of three issues of a Bulletin of International Associations.

The founders of the UIA suffered a series of disappointments and difficulties. After the State took back the Cinquantenaire Palaces, La Fontaine and Otlet were obliged to move with their collections and archives to other premises, and to continue their work in extremely uncomfortable conditions. The German occupation was to complete the disaster when the military government destroyed 63 tons of periodicals.

In the 1930s much of the UIA's documentary activities were taken over by the League of Nations. The League paid formal tribute to the UIA's work of documentation and coordination of effort as a "vast enterprise of international intellectual organization, characterized by the breadth of its conception and design".

The 7th (and last) World Congress of International Associations was held. Owing to conflicting initiatives, financial difficulties and too many projects that aroused some alarm, La Fontaine and Otlet became more and more isolated in their work. With extraordinary tenacity, they kept on in spite of all. Right up to the outbreak of World War II they continued with their wholly admirable documentary work.

With the support of the League of Nations, the UIA created an International University. Fifty professors from 11 countries gave lectures to about 100 students from over 10 countries.

The League of Nations was established on 10 January, following intensive lobbying for its creation by the UIA. Henri La Fontaine served as rapporteur of the League of Nations commission whose work resulted in the establishment of the International Institute of Intellectual Cooperation.