This paper deals with the treatment of constructed neologisms in a machine translation system. It focuses on a particular issue in Romance languages: relational adjectives and the role they play in prefixation. Relational adjectives are formally adjectives but are semantically linked to their base-noun. In prefixation processes, the prefix is formally attached to the adjective, but its semantic value(s) is applied to the semantic features of the base-noun. This phenomenon has to be taken into account by any morphological analyser or generator. Moreover, in a contrastive perspective, the possibilities of creating adjectives out of nouns are not the same in every language. We present the special mechanism we put in place to deal with this type of prefixation, and the automatic method we used to extend lexicons, so that they can retrieve the base-nouns of prefixed relational adjectives, and improve the translation quality.