The present study examines differences in syntactic complexity in English writing among advanced and upper intermediate undergraduates by means of quantitative measures of syntactic complexity. Participants were 250 Spanish undergraduates enrolled in a Degree in Modern Languages. A total of 121 students had an upper intermediate level (B2 level according to the CEFR) and 129 had an advanced level (C1 level according to the CEFR). Essays were evaluated by quantitative measures gauging different aspects of L2 complexity. Results indicate that the complexity measures chosen can capture significant differences in writing proficiency when comparing different proficiency levels. The scores on the general quality of the writings and on all syntactic complexity measures increased from B2 to C12 and for all complexity measures the increase was statistically significant.