The pantropical genus Caesalpinia sensu lato contains about 120 to 130 species, of which 70 to 75% are neotropical. The genus belongs in the Caesalpinia group (Caesalpinioideae: Caesalpinieae) of Polhill and Raven (1981). A morphological cladistic analysis of the Caesalpinia group demonstrates that the traditional circumscription of the genus Caesalpinia is polyphyletic: some species are more closely related to other genera in the group than to each other. The Poincianella and Erythrostemon elements of Caesalpinia sensu lato, selected for more detailed study, have been united and the resulting, strictly neotropical group, has been expanded to include several South American taxa previously placed in other infrageneric groups of Caesalpinia sensu lato. The Poincianella- Erythrostemon group has been revised and 56 taxa in 47 species are currently recognised. These have a geographical range from south eastern U.S.A. to southern Chile and most species have a predilection for semi arid areas. A key to species identification, full descriptions and specimen citations are provided; most species are illustrated and all are mapped. One new species, Caesalpinia coccinea, discovered during this research, has already been published (Lewis and Contreras, 1994). Eight other new taxa and two new combinations presented here are to be effectively published in a future volume of Kew Bulletin. Sections on morphology, seed chemistry, biogeography and floral biology discuss the relationships between the Poincianella-Erythrostemon group and other infrageneric groups of Caesalpinia s.l. A survey of floral secretory structures in Caesalpinia sensu lato is presented in Appendix 1 and an account of the sexual systems of two species of the Poincianella-Erythrostemon group in Appendix 2.