The study aims to analyze the relationship between six measures of syntactic complexity (MCS) and the writing quality in adolescents. 160 narrative and explanatory texts written by 80 7th and 11th grade high school students from two schools in Santiago were analyzed. Through a correlational analysis, the results show that the text length is the measure most related to quality in all groups, while other measures of density and syntactic complexity are only related to performance in grade 11. In addition, it was verified that this relationship is also affected by the discourse genre (more MCS in explanations) and the SES (more phrasal resources in high socioeconomic status –SES–, and more clause resources in medium SES). More research is needed on the relationship between syntactic complexity and the quality of writing, a topic of great pedagogical and developmental interest that has not been much studied in Chile.