I am updating the official registries section of my business sources listings (http://www.rba.co.uk/sources/registers.htm) and there are changes to the entries for Luxembourg and Belgium.
The Registre de Commerce et des Sociétés – Accueil (http://www.rcsl.lu/) is the official register of companies and associations in Luxembourg. The search options are limited to company name or number and the interface is in German and French. Searching and company name, address and contact details are free of charge. Documents are priced.
Legilux Sociétés et Associations has more search options at http://www.legilux.public.lu/entr/search/index.php and it provides a history of the documents filed by a company. This is a free service but for the documents themselves you have to go back to the Registre de Commerce et des Sociétés where there is a charge per document.
In Belgium the KBO Public Search (http://economie.fgov.be/nl/ondernemingen/KBO/Pubd/PuS/)
enables you to search for public information on every registered active enterprise and establishment. Search by company number, name, branch number or name, address and municipality. Each record provides name, company number, activities, address, contact details and links to other sites for official documents and annual reports. The search interface is available in Dutch and French. The information is in Dutch or French and is free.
The CBSO (Central Balance Sheet Office) section of the National Bank of Belgium (http://www.bnb.be/) has the accounts of companies, associations and foundations active in Belgium. The search interface is available in Dutch, French, German and English and the services is free.
Many thanks to Inez de Bois for the information and updates.