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.

Otlet and La Fontaine were granted space in the left wing of the Palais du Cinquantenaire in Brussels to house the Palais Mondial. They conceived it to be a monument to all the material and intellectual glories of the universe, a sacred place of inspiration of grand ideas and noble activities, a treasury of all works of spirit, brought there as a contribution to science and to universal organization.

The UIA began lobbying for a Belgian law providing special legal facilities for international associations. The law was adopted on 25 October 1919, and the UIA was registered as an international association with scientific aims under the new Belgian law on 2 July 1920.

The work connected with the formation of international associations was carried on actively and increasingly up to the beginning of World War I, when over 500 were in existence. By 1914, the UIA had federated 230 international non-governmental organizations, or around half the total number existing at that time. But at the outbreak of hostilities, the activities of associations necessarily slackened, and in some cases ceased altogether.

At the First World Congress of International Organizations in Brussels, representatives from 137 international civil society bodies and 13 governments formally agreed to transform the Central Office into the Union of International Associations. It was to provide services including management of relations between international associations, study of questions of common interest, creation of new organizations, international instruction, management of publications and documentation, and other general services.

In 1910, Otlet and La Fontaine first envisioned the Palais Mondial (later the Mundaneum), which aimed to bring together all leading international institutions in the world, to serve as a central repository for the world's information, to radiate knowledge to the rest of the world, and to construct peace and universal cooperation.

Collaboration began with the Institut International de Bibliographie and the Institut International de la Paix on the 1908 and 1909 editions of the Annuaire de la Vie Internationale, the precursor to the Yearbook of International Organizations. It was published from 1910 until 1911 with the support of the Carnegie Endowment for International Peace. It contained descriptions of approximately 150 international organizations.