Migration and HIV research in sub-Saharan Africa has focused on HIV risks to male migrants, yet women's levels of participation in internal migration have met or exceeded those of men in the region. Moreover, studies that have examined HIV risks to female migrants found higher risk behavior and HIV prevalence among migrant compared to non-migrant women. However, little is known about the pathways through which participation in migration leads to higher risk behavior in women. This study aimed to characterize the contexts and processes that may facilitate HIV acquisition and transmission among migrant women in the Kisumu area of Nyanza Province, Kenya. We used qualitative methods, including 6 months of participant observation in women's common migration destinations and in-depth semi-structured interviews conducted with 15 male and 40 female migrants selected from these destinations. Gendered aspects of the migration process may be linked to the high risks of HIV observed in female migrants - in the circumstances that trigger migration, livelihood strategies available to female migrants, and social features of migration destinations. Migrations were often precipitated by household shocks due to changes in marital status (as when widowhood resulted in disinheritance) and gender-based violence. Many migrants engaged in transactional sex, of varying regularity, from clandestine to overt, to supplement earnings from informal sector trading. Migrant women are at high risk of HIV transmission and acquisition: the circumstances that drove migration may have also increased HIV infection risk at origin; and social contexts in destinations facilitate having multiple sexual partners and engaging in transactional sex. We propose a model for understanding the pathways through which migration contributes to HIV risks in women in high HIV prevalence areas in Africa, highlighting potential opportunities for primary and secondary HIV prevention at origins and destinations, and at key 'moments of vulnerability' in the migration process.