GaPyAs1-y/GaAs, GaAsySb1-y/GaSb and GaSbyAs1-y/GaAs superlattices (SLs) grown by MBE, by exposure of GaAs to phosphorus and antimonide fluxes, and by exposure of GaSb to an arsenic flux, respectively, have been investigated. The focus is on the abruptness of interfaces and understanding the mechanisms associated with anion incorporation and exchange. In the case of the Sb flux interaction with the GaAs surface, the Sb segregation at the GaAs surface inhibits anion exchange. For the case of As over GaSb reactions, anion exchange results in the formation not only of the ternary alloy GaAsySb1-y, but also of isoelectronic compounds AsSbx that segregate at the GaSb/GaAs interface. In the case of the P flux interfaction with the GaAs surface, fast in-diffusion of P results in graded GaPyAs1-y layer formation.