We have used a modified molecular beam epitaxy method, in which the active species of nitrogen is derived from an Oxford applied research rf activated plasma source, to grow AlN, GaN, InN, Al(AsN) and Ga(AsN). In this paper we discuss, for the first time, the growth kinetics of both binary nitrides and "ternary alloys" in the light of both our observations and those of other groups working in this area and their relevance to the properties of films grown under various conditions.