The electron probe has been used to measure the composition of schreibersite inclusions in 26 iron meteorites. The range of nickel concentrations found was from 14 to 50 per cent (by weight), the lowest values being for large inclusions and the highest for small ones. Analyses of rhabdites in Canyon Diablo and Coahuila were also carried out. Rhabdite showed a range of nickel contents also correlated with grain size. The range of compositions of both forms of (Fe, Ni) 3P is the result of disequilibrium due to limited diffusion at low temperatures. The electron probe data have been interpreted in terms of the phase diagram of the iron-nickel-phosphorus system. It is concluded that all phosphide precipitation took place in the solid state, after the start of the γ-α. transformation which gives rise to the Widmannstätten structure. The smallest and most nickel-rich schreibersite grains nucleated in taenite at kamacite/taenite interfaces, while rhabdite is the result of homogeneous nucleation of (Fe, Ni) 3P in kamacite supersaturated with phosphorus at low temperatures (400–500°). Both rhabdite and schreibersite nucleation appear to have taken place simultaneously in this temperature range, although most schreibersite nucleated at higher temperatures. The morphological difference between the two forms of (Fe, Ni) 3P is interpreted as being the result of different nucleation mechanisms, which probably result in different phosphide/kamacite lattice orientation relations. Kamacite was found to contain 0.05–0.10 per cent phosphorus, which is consistent with a “freezing-in” temperature of about 350° on the one-atmosphere phase diagram. The present results are compatible with a history of very slow cooling at low pressures (not more than a few kilobars), but more quantitative information on phase equilibria in the iron-nickel-phosphorus system will be required before more positive deductions from the electron probe data can be made.