Despite the massive expansion of higher education in China since 1998, the cohort trends of urban and rural "hukou" holders in college attendance have widened sharply. Prevailing explanations emphasize the advantages of urban students over rural students in school quality and household financial resources. We propose the structural exclusion hypothesis that underscores the unintended consequences of a state policy: the urban concentrated expansion of vocational upper secondary education. This policy makes the expanding opportunity inaccessible for most rural students but helps lower-achieving urban students remain in the "pipeline" for college. We conduct a crucial test of these explanations by linking provincial-level enrollment statistics with individual-level models of the urban-rural trends in college attendance. The data are drawn from the 2006 Chinese General Social Survey and official statistics for 28 college admission districts over 14 college admission cohorts (1989-2002). Findings suggest that the rising urban advantage originates from the virtually exclusive increase in opportunities for vocational education among urban students. As vocational education is mainly an option for lower-achieving students, the expansion of vocational education most benefits lower-achieving urban students. The widening differences between urban and rural "hukou" in college attendance therefore reflects the advantage given "marginal" urban students in access to vocational schools.