The articles 12 and 13 of Protocol (No 7) on the privileges and immunities of the European Union lay down the tax regime for officials of the European Institutions.
Officials of the European Institutions living in Belgium are deemed to have kept their residence in the EU Member State where they had their tax residence before coming to Belgium. That is not necessary their country of nationality. This rule applies to income tax and inheritance tax. And if they are a national of a country that has a wealth tax, they are liable to that wealth tax. They can also invoke the double tax treaties concluded by that country.
This means that
An official who earns income from an activity outside the European Institiutions may be liable to income tax in Belgium as a non-resident.
This provision also applies to the spouse of an official (unless that spouse has earnings in Belgium), and to the dependent children that are in the care of the official.
For a full guide on the taxation of EU officials in Belgium, click here.