ABSTRACT Histone H3 tail modifications are among the earliest chromatin changes in the X-chromosome inactivation process. In this study we investigated the relative profiles of two important repressive marks on the X chromosome: methylation of H3 lysine 9 (K9) and 27 (K27). We found that both H3K9 dimethylation and K27 trimethylation characterize the inactive X in somatic cells and that their relative kinetics of enrichment on the X chromosome as it undergoes inactivation are similar. However, dynamic changes of H3K9 and H3K27 methylation on the inactivating X chromosome compared to the rest of the genome are distinct, suggesting that these two modifications play complementary and perhaps nonredundant roles in the establishment and/or maintenance of X inactivation. Furthermore, we show that a hotspot of H3K9 dimethylation 5′ to Xist also displays high levels of H3 tri-meK27. However, analysis of this region in G9a mutant embryonic stem cells shows that these two methyl marks are dependent on different histone methyltransferases.