Short answer. It must be stated in plain terms. Article 865 of the Civil Code requires that every fideicommissary substitution be expressly made in order to be valid. An implied or inferred substitution has no legal effect — if the will does not clearly impose the obligation to preserve and transmit the inheritance, there is no fideicommissary arrangement.
What the law says
Every fideicommissary substitution must be expressly made in order that it may be valid.
Civil Code, Article 865 — It Must Be Express. Read the full provision →
Why the law requires express language
A fideicommissary substitution places a significant burden on the first heir: they receive the inheritance but are legally obligated to preserve it and eventually hand it over to someone else. Because this restriction is so serious — limiting what the first heir can do with the property and binding them to an obligation of stewardship — the law does not allow it to be inferred from ambiguous language. The will must say so in terms that make the obligation unmistakable. If a court would need to speculate about whether the testator intended a fideicommissary arrangement, the arrangement does not exist.
What happens once the arrangement is validly created
When a fideicommissary substitution is properly and expressly stated, the law defines clearly what the first heir (the fiduciary) must deliver to the second heir: the inheritance itself, minus only those deductions that arise from legitimate expenses, credits and improvements. The fiduciary is not entitled to keep proceeds from the property or whittle down what the second heir receives through ordinary management costs. Deductions are allowed, but only from the specific categories the law names. The testator can also provide for a different arrangement, but only by saying so expressly in the will.
How to tell if a will creates a fideicommissary substitution
Look for language that clearly imposes two obligations on the first heir: first, to preserve the inheritance (not dispose of it freely), and second, to transmit it to a named or identifiable second heir. Both elements must appear in the will. A clause that simply gives an heir property, or that expresses a wish that property eventually pass to another person, may fall short. Wishes and hopes are not the same as legally imposed obligations. If the will is ambiguous on either element, the fideicommissary character of the disposition is in question.
Practical implications for reading a will
If you are a first heir (fiduciary) and you are unsure whether you are bound to preserve and transmit the property, the expressness requirement actually works in your favor: if the will does not clearly say so, the restriction may not bind you. Conversely, if you are the intended second heir and the will is vague, the arrangement may not be enforceable against the first heir. Reading the will carefully — and getting professional help interpreting it — is essential before anyone makes decisions about the inherited property.