GaAs (001), CaP (001), and InP (001) surfaces were exposed to N-2 and N-2-H-2 plasmas in order to investigate the substrate nitridation process. In situ realtime ellipsometry was used to assess the nitridation self-limiting kinetics. Ex situ x-ray photoelectron spectroscopy and atomic force microscopy fingerprint the surface composition and morphology, respectively. For GaAs, N-2 plasma nitridation forms a GaN layer whose thickness is limited by the As segregation at the GaAs/GaN interface For GaP and InP substrates, a phosphorous nitride capping layer was formed which inhibits further nitridation. For all substrates, the H-2 addition to N-2 plasmas results in an increase of the nitridation depth since hydrogen favors the desorption of AsHx and PHx species thereby reducing the inhibition effect of both the As segregation and phosphorus nitride formation.