The conversion, as a way of correcting the causes of nullity of the civil legal act, which consists in capitalizing in a transformed manner the manifestation of valid will expressed in a civil legal act struck by nullity in a subsequent civil legal act, has applicability in various matters of civil law. This paper focuses on the identification and analysis of conversion hypotheses in the field of civil obligations, which can be deduced from the norms of the Civil Code. Thus, the author noted that the manifestation of will, which, void as alienation, may be worth pre-contract of alienation, and to this hypothesis of conversion the author attributed the conversion of a legal act of disposition on a good, concluded in violation of the prohibition of alienation and with the violation of the inalienability clause, as well as the conversion of legal alienation of a common property made by one spouse without the consent of the other spouse. There is also a case of conversion of the legal act in the situation in which an act of constituting an autonomous personal guarantee struck by absolute nullity will be able to be valid as an act of constitution of a surety. The conversion can also take place in case of nullity of the bill of exchange, which can be valued as a writing certifying a claim (confirmation of debt).