All member states are held by EU Regulation 1408/71 determining which social security system applies.

Employees who are simultaneously employed in two or more member states, which is the case for a salary split, are subject to the social security legislation of the home member state if they perform part of their activities in this state or if they are attached to more than one employer who is registered or domiciled in the territory of several member states (article 14, section 2, b, i).

An employee who works simultaneously in Belgium and in another member state as employee remains fully covered in Belgium, provided he maintains his habitual abode in Belgium and performs a non-marginal part of his activities in Belgium (i.e. at least 5 hours a week). Coverage is ensured for as long as the split employment lasts. A split employment is therefore neutral from a social security point of view.

Those who perform a self-employed activity in Belgium and in another Member State are subject to a similar regulation (home country principle). Given the ceiling on social security contributions for the self-employed in Belgium, this may be advantageous.

When an employee is subject, based on the EU Regulation, to Belgian social security for the whole of his activities performed in Belgium and in another member state, a number of formalities must be fulfilled:

	The employee (or the employer, for the employee’s account) must ask for a form E101 from the Belgian social security administration under article 14.2.b.i of the EU Regulation. First a questionnaire must be completed. Form E101 serves as evidence that the employee continues to be covered in Belgium and is exempted from social security contributions in the other Member State.
	The employee must ask for a form E128 from his sickness fund in order to be covered for health care in the other member state where he performs activities.

The foreign enterprise must be affiliated to the Belgian National Social Security office (RSZ/ONSS). If the foreign company belongs to the same group as the Belgian company, it is possible for the foreign company to be affiliated to the Belgian social security scheme under the social security number of the Belgian firm. However this means that both firms should sign a statement transferring responsibility for the Belgian social security contributions on the foreign part of the employee’s salary from the foreign employer.

The practical consequence of such a statement is that the Belgian company pays the total social security contributions due on the Belgian and foreign parts of the salary and recharges the foreign firm for the social security contributions to be paid on the foreign salary.

In addition, the foreign company will have to observe the compulsory affiliations (e.g. to a child benefits fund) and take out an industrial injuries insurance (concluded at the latest when the employee enters into service) applicable to each new employer who employs personnel who is subject to the Belgian social security scheme. This is preferably done through a social secretariat.