This paper is a study of the NofN structure nouns, which may be described as compound nouns by juxtaposition. 1500 word sequences are observed, aiming at contributing to the formal description of the Brazilian Portuguese lexicon, and defining the identification criteria of a compound noun with such a structure. The general criterion is based on the concept of semantic non- compositionality. The tests are made from the syntactic and semantic properties existing in the relationship between the elements that constitute the nominal group, showing the differences between a free nominal group and a compound nominal group. Among such properties, the following can be pointed out: distributional blockage, inseparability, lexical insertion, N2 erasing, N2 substitution, and gender and number variations. Such description proves to be useful in the sense that a set of lexical units delimitation rules and criteria has been defined, constituting a basis for the incorporation of new items to the lexicon. Finally, the incorporated entries` properties received a formal representation, which resulted in the creation of an electronic dictionary that can be used in electronic processes.