This dissertation examines the colonies of Sierra Leone and Liberia, settlements established by British and American anti-slavery societies respectively. It looks at cultural institutions, settler identification, commercial networks, and missionary activity between Liberia's founding in the 1820s and the beginning of the American Civil War and British annexation of Lagos in 1861. This dissertation argues that the development of settler society in Sierra Leone and Liberia led to the formation of certain types of relationships between the colonies and between the colonies and the metropoles that contributed to the perception of the viability of colonization as an anti-slavery intervention tool in the metropolitan context. The settlers were crucial in developing the concept of 'civilization, commerce, and Christianity' as a set of measures for abolishing the slave trade, but their ability to pursue these measures was also affected by the changing state of anti-slavery activism in the metropoles. -- This dissertation uses a comparative approach to the colonies in order to fill gaps in the current literature, which neglects the interactive nature of the colonial relationships, and therefore misses a crucial factor in explaining the divisions in and between the antislavery societies. Despite the British and American anti-slavery colonization organizations' similar goals, they were frequently unable to cooperate or share resources, particularly in slave trade suppression, or in support of West African anti-slavery colonization. This was in part because of commercial, territorial, and anti-slavery 'humanitarian expansion' by settlers in Sierra Leone and Liberia which fostered rivalry between the two settler societies and their metropolitan supporters.