Molecular studies of the guenons suggest that the arboreal Cercopithecus species form a monophyletic group within the tribe Cercopithecini. However, the evolutionary relationships among these arboreal congeners remain poorly resolved. The present work marks the first attempt to reconstruct the history of this group through the phylogenetic analysis of long nuclear sequences. We surveyed 19 guenons and seven outgroup taxa for a approximately 9.3kb fragment of X-chromosomal DNA homologous to a portion of human Xq13.3. Parsimony and maximum likelihood analyses of these sequences consistently recover two strongly-supported patterns within the arboreal Cercopithecus clade: (1) a clustering of members of the cephus and mitis species groups, and (2) a monophyletic aggregate including the mona, neglectus, and diana species groups. Although guenons occasionally hybridize in the wild, interbreeding forms of different species groups do not cluster together as sister-taxa in the X-chromosomal tree, suggesting that the two clades inferred here are not reticulate patterns due to recent gene flow. These clades are most likely the result of either ancestral hybridization or true phylogenetic history. We advocate the latter explanation because the same two aggregates (cephus/mitis and mona/neglectus/diana) are recovered, albeit with weak support, by a number of earlier analyses. Finally, X-chromosomal divergence dates are estimated for a number of nodes in the guenon radiation. The divergence of guenon and papionin lineages at 11.5 (+/-1.3) MYA appears to be a particularly robust estimate since it is inferred from both mitochondrial and X-chromosomal studies, each using different fossil calibration points.