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.

Throughout the pre-1914 period, Belgium was the main host country of the international mouvement, welcoming up to one-third of the international organizations. The Central Office of International Associations was founded in Brussels to enhance collaboration between the organizations, and to serve as a centre for documentation.

From 1903, the idea of a universal synthesis of knowledge led Otlet and La Fontaine to broaden their scope, previously focused on scientific publications. Newspapers, graphic materials and other information sources were treated in addition. To accommodate this new material, other international institutions were created. This period also saw the birth of 20th century internationalism, and Otlet and La Fontaine were fervent internationalists.

Otlet and La Fontaine established the Répertoire Bibliographique Universel (RBU), an ambitious attempt at developing a master bibliography of the world's accumulated knowledge, and at providing information for retrieval on anything of note published anywhere in the world. By the late 1930's, the RBU had grown to over 12 million entries.