The compositions of kamacite, taenite and fine plessite in a number of iron meteorites have been investigated with the electron probe. The results are discussed in terms of the nickel-iron phase diagram. Kamacite has been found to be essentially homogeneous, except for a nickel depletion in the immediate vicinity of kamacite/taenite interfaces, while taenite is highly inhomogeneous. The nickel content of kamacite and taenite at the interface between them has been measured in 17 specimens, and the concentration of nickel in kamacite away from such interfaces in 26 specimens. From these results it is concluded that the solubility of nickel in kamacite must decrease below about 450-500°C, which is not in accordance with the experimental data on the iron-nickel system. This is probably due to the effect of phosphorus or other minor constituents, or inaccuracy in the phase equilibrium data. It is concluded that variations in the kamacite and taenite compositions are due to differences in the cooling rate of the material of which iron meteorites are constituted. This is confirmed by measurements of nickel concentrations in fine plessite and phosphorus in kamacite. This variability in cooling rate appears to be at least partly a function of the Ga/Ge content.