Few studies have addressed vocabulary quality in developing writing skill in Spanish. Even less addressed it within the Chilean educational system. The specific objective of this study was to characterize, using a comprehensive set of indicators, the quality of the vocabulary produced by Chilean 4th grade students. Based on a national writing survey, a sample of 2056 texts written by 685 students was collected (narrative, persuasive, and informative texts). Current literature defines lexical quality as a composite of diverse factors that, while distinct, are interrelated. To represent the properties of the vocabulary, a set of indicators were selected: (a) lexical diversity; (b) lexical sophistication; and (c) lexical density. Using multilevel modeling (students and schools as levels 1 and 2) to explain a global writing score we found that diversity was a significant determinant for narrative and persuasive texts, density was a significant determinant for the three genres and sophistication was a significant determinant for narrative and expository text. In addition, indicators related to gender and socioeconomic conditions were only significant determinants of narrative stories. The parts of speech most often used also varied according to the purpose of each text. In all genres, words had a short extension and were very sensitive to the input presented in the stimuli. These results imply a significant challenge to this education system: how to promote the development of vocabulary in all children in order to support language learning.