Short answer. No, not the principal. If a credit belongs to one spouse, the installment payments on the principal collected during the marriage stay that spouse's exclusive property. But interest that falls due during the marriage on that same credit belongs to the conjugal partnership.
What the law says
the sums which may be collected during the marriage in partial payments or by installments on the principal shall be the exclusive property of the spouse. However, interests falling due during the marriage on the principal shall belong to the conjugal partnership.
Family Code, Article 119 — Credit Payable in Installments. Read the full provision →
The starting point: whose credit is it
Article 119 applies where "an amount or credit payable within a period of time belongs to one of the spouses." This is the necessary starting fact — the article is not about a debt owed jointly to both spouses, but about a receivable that already belongs to just one of them, perhaps because it arose before the marriage or from that spouse's exclusive property. Everything the article says about installments and interest is built on that premise.
Why the principal stays exclusive
The article's core rule is that "the sums which may be collected during the marriage in partial payments or by installments on the principal shall be the exclusive property of the spouse." Collecting the debt in installments during the marriage does not convert it into conjugal property. Each installment is simply a piece of the same credit that already belonged to that spouse before it was collected — the timing of collection, during the marriage, does not change whose property it is.
Why interest is treated differently
The article draws a clear line between the principal and any interest: "however, interests falling due during the marriage on the principal shall belong to the conjugal partnership." Interest is treated as fruit generated by the credit during the marriage, rather than as part of the credit itself, and the Family Code's general approach is to treat fruits and income arising during the marriage as conjugal. So while the underlying principal stays with the owning spouse, whatever interest accrues on it while the marriage is ongoing goes to the partnership.
The practical split this creates
In practice, this article requires separating any collection into two components: how much of it is principal, and how much is interest. The principal portion, however it is collected — in a lump sum or in installments — remains the exclusive property of the spouse it belonged to. The interest portion, if it falls due while the marriage subsists, is conjugal. Treating the whole collected amount as one or the other, without separating principal from interest, does not match what this article actually provides.