The conventional approach to growth of the nitride films GaN, AlN, InN, and their alloys by rf plasma molecular beam epitaxy uses metal-rich surface conditions due to improved material quality compared to nitrogen-rich conditions. The surface metal may incorporate into the growing film, act as a surfactant, and/or react with the underlying film or substrate. Using a simple chemical exchange reaction model and tabulated thermodynamic data at molecular beam epitaxy growth temperatures the predicted preferential incorporation series on the column III site under metal-rich conditions is found to be Al>B,Be,Si, Mg>Ga>In,Fe. This series is consistent with the observed ternary growth behavior and surfactant order. The series is also consistent with silicon migration in AIN but not GaN, sharper beryllium transitions in GaN than AIN, the significant migration of iron in GaN, and the reactivity of AIN nucleation layers with SiC surfaces. The model is used to predict boron incorporation under metal-rich conditions in BGaN and BAlN and should prove useful as a tool in predicting the incorporation behavior of other cations during metal-rich epitaxial growth of nitride films and possibly other materials. (c) 2007 American Vacuum Society.