Sources of information
Only the non-profit association is discussed here. Other forms of associations may differ in administrative procedures.
Information brochure
The Federal Public Service of Justice has a fairly good information brochure (FR) on the creation of an association following the 2002 in-depth revision of the Law of 27 June 1921.
Searching publications of association in the Belgian Official Gazette
Since 01/07/2003, the filing formalities have been dealt with by the registries of the trade courts, and the publications filed can be found in the same database of the Belgian Official Gazette (FR) as that of the companies.
There is also a reference database for publications of associations for the period 01/01/1981 to 30/06/2003, where you can find the reference of the publication in the Belgian Official Gazette, and as from 01/12/1998 you can view the full text online.