Method for coprecipitation (or simultaneous precipitation) of at least one actinide in oxidation state (IV) with at least one actinide in oxidation state (III), wherein: a solution i.e. mixture of actinide(s) in oxidation state (IV) and actinide(s) in oxidation state (III) is prepared by adding to it a singly charged cation whose presence makes it possible to stabilize the aforementioned oxidation states in the mixture, or a singly charged cation which does not act to stabilize the aforementioned oxidation states in the mixture; a solution containing oxalate ions is mixed with the said mixture of actinides in order to carry out coprecipitation, i.e. simultaneous precipitation, of the said actinides in oxidation states (IV) and (III) and a fraction of the singly charged cation. According to another embodiment, a solution i.e. mixture of actinide(s) in oxidation state (IV) and actinide(s) in oxidation state (III) is prepared and a solution containing oxalate ions and a singly charged cation is added to the said mixture of actinides in order to carry out the coprecipitation.Method for preparation of mixed compounds of actinides by calcining the precipitate which is obtained.These mixed oxides such as mixed oxides, carbides or nitrides can be used especially in the fabrication of nuclear fuel, in the fabrication of transmutation targets or for the stable packaging of nuclear materials.