In this paper we examine differences in upper secondary school attendance among subgroups of the population in Thailand. We ascertain where inequalities continue to exist and where they have been mediated. We analyze data from samples of the 1990 and 2000 Thai censuses. We find that the gender gap favoring boys has closed at the national level and that girls now transition to upper secondary school more often than do boys. Inequalities still exist according to socioeconomic background of families. Rural-urban differences have narrowed considerably since 1990, but they continue to be observed, particularly in certain parts of the country. Similarly, differences between Muslims and others appear to have lessened over the last decade, but Muslims continue to be disadvantaged compared to others, particularly in the South. (Contains 1 figure, 6 tables and 1 footnote.)