Short answer. On six grounds, and no others. Article 839 disallows a will for defective formalities, insanity or mental incapacity at the time of execution, force or duress or threats, undue and improper pressure and influence, a signature procured by fraud, and mistake as to what the document was.

What the law says

The will shall be disallowed in any of the following cases: (1) If the formalities required by law have not been complied with; (2) If the testator was insane, or otherwise mentally incapable of making a will, at the time of its execution;

Civil Code, Article 839 — Grounds to Disallow a Will. Read the full provision →

A closed list, and unfairness is not on it

Article 839 begins: The will shall be disallowed in any of the following cases: (1) If the formalities required by law have not been complied with; (2) If the testator was insane, or otherwise mentally incapable of making a will, at the time of its execution; and continues through force or duress, undue and improper pressure and influence, a signature procured by fraud, and mistake about the nature of the document. Nothing in the list is about the contents being unjust. A parent may prefer one child over another; that is a complaint about the legitime, not a ground to disallow the will.

Formalities are the ground that succeeds most quietly

The first ground is the least dramatic and the most effective, because it is proved from the document rather than from memory. Article 805 requires a notarial will to be subscribed at the end by the testator and attested and subscribed by three or more credible witnesses in the presence of the testator and of one another. Article 806 requires it to be acknowledged before a notary public by the testator and the witnesses. Article 808 adds that where the testator is blind, the will must be read to him twice — once by a subscribing witness and again by the notary. A gap in any of these is objective.

Capacity, coercion and influence are three different arguments

Ground two is measured strictly at the moment of execution, so illness, old age, or a later decline does not by itself establish incapacity on the day the will was signed. Grounds three and four are often confused. Force, duress, fear or threats describe pressure that overrides the will outright. Undue and improper pressure and influence describes something subtler — a beneficiary or another person substituting their intention for the testator's, typically through control over an isolated or dependent person. The fifth and sixth grounds are narrower still, aimed at a signature obtained by fraud or affixed without knowing the document was a will.

What disallowance actually produces

Winning is not the same as inheriting more. Article 960 provides that legal or intestate succession takes place where a person dies with a void will or one that has lost its validity, so the estate is then distributed by the rules the law supplies — which may or may not improve a particular sibling's position. Before committing to a contest, get a certified copy of the will with its attestation clause and notarial acknowledgment, the medical records around the date of execution, and a clear account of who arranged the signing, who was present, and who chose the notary.

Cases citing this provision

These Supreme Court decisions cite the provision above. We list them so you can read them yourself; the summaries of what each decided are not ours to give.

Related provisions

Note. Statute text quoted on this page is reproduced from the official enactment and is linked to the full provision. The explanation around it is general legal information from Vivas & Nobles Law Office, not legal advice. Whether it applies to your situation depends on facts only a lawyer reviewing them can assess.