The purpose of this study is to analyse how vocabulary depth develops in the early years of primary school and to ascertain the effect it has on the quality of the texts produced by the students. A total of 332 Spanish students from the first to fourth grades participated by doing a synonyms/antonyms task to evaluate their vocabulary depth, along with several tasks in which they wrote descriptive texts to evaluate text quality. Vocabulary depth was analysed using a four-dimensional system created ad hoc: change in lexeme, fitting the meaning requested, preservation of syntactic category and degree of conventionality. The text quality indicators were productivity, subordination rate and text organization. The results revealed that both vocabulary depth and the quality of the texts produced improved significantly by grade. The study also found that this type of lexical knowledge contributes significantly to the quality of the texts evaluated. Finally, this article discusses the possible teaching implications of these findings. [Translation from Spanish by Mary Black.]