Definition
In civil law, a person who assumes the debt of another and becomes solely and originally liable for it in the original debtor's place, by agreement with the creditor. The expromissor does not stand beside the original debtor as a guarantor; the original debtor is discharged entirely, and the expromissor steps into the debtor's position as the primary — and only — obligor. The transaction by which this substitution is accomplished is expromission, a form of novation by change of debtor.
Common Confusion
The expromissor is frequently compared to a surety, but the two roles are legally distinct in a fundamental way. A surety remains jointly liable with the principal debtor — both obligors exist simultaneously, and the creditor may pursue either. The expromissor, by contrast, replaces the original debtor entirely. The original debtor is released, and the expromissor alone bears the obligation. The mechanism is novation, not guaranty. Researchers who encounter these terms in historical sources dealing with Roman law or civil law jurisdictions should not treat them as interchangeable with common-law suretyship concepts.
Why It Matters in Research
EXPROMISSOR is a term of Roman and civil law origin and appears almost exclusively in sources treating civil law systems — Roman law treatises, Louisiana legal materials, Scottish law, and Continental European jurisprudence. Researchers working in the common law tradition will rarely encounter this term in operative legal documents, but will find it frequently in 19th-century treatises that drew on Roman law to explain novation and debt substitution.
The critical research trap: historical common-law sources sometimes use "novation" loosely to describe transactions that do not involve a full discharge of the original debtor. When a source uses EXPROMISSOR or EXPROMISSION, however, the discharge of the original debtor is definitionally present — it is not optional or implied. Researchers should not assume this precision holds when civil law vocabulary is borrowed into common-law contexts without careful adaptation.
In the Law Mind corpus, this term connects most directly to entries on NOVATION, DELEGATION (in the civil law sense of substituting a debtor), and EXPROMISSION. It also bears on SURETYSHIP entries, where the contrast between joint liability and substituted liability is drawn. Louisiana sources in the corpus are particularly likely to deploy this terminology in its technically precise civil law sense, given Louisiana's mixed legal heritage.
Historical Dictionary Support
The historical dictionaries agree on the core meaning and the critical distinction from suretyship, though they vary in emphasis.
Black's (both editions) defines the expromissor as one who assumes another's debt and becomes solely liable by stipulation with the creditor, identifying the transaction as novation and contrasting it with the surety's joint liability. Both editions cite Mackeld. Rom. Law (§ 538 in the 1st edition, §638 in the 2nd — a discrepancy worth noting when verifying citations to Mackeldey's Handbook of the Roman Law).
Bouvier adds a nuance the other sources underemphasize: the expromissor assumes liability "whether the latter were obligated or not," suggesting the mechanism operates even where the original debtor's liability was contingent or defective. This point aligns with Roman law doctrine on delegation and is worth attending to when the source obligation is itself contested.
Burrill is the most technically precise of the historical sources, explicitly linking the expromissor to the civil law process of delegation (delegatio) and noting that the expromissor is "substituted as a debtor" — language that foregrounds the structural mechanics of the transaction. Burrill also references Heineccius (Heinecc. El. Jur. Civ.) and Calvin's Lexicon, sources that researchers working in Roman law will recognize as reliable secondary authorities.
None of the historical dictionaries adequately address how expromission interacts with defenses the original debtor might have had against the creditor, a point of some complexity in Roman law sources and worth independent investigation when the issue arises.
Jurisdictional Note
This term is operative primarily in civil law jurisdictions and in Louisiana. In common law jurisdictions, the functional concept (substitution of one debtor for another with discharge of the original) exists but is handled through novation doctrine without the Roman law nomenclature. Researchers should not expect English or most American courts to use or recognize EXPROMISSOR as a term of art outside of academic or comparative law contexts.
Encyclopedia Cross-Reference
Law Mind Encyclopedia — Novation
Law Mind Encyclopedia — Suretyship
Law Mind Encyclopedia — Delegation (Civil Law)