For age determinations it is essential to obtain a reasonable range of parent-daughter physical-chemical fractionation between whole rock samples for the determination of whole rock isochrons, and between constituent phases for the possible determination of internal isochrons. For the application of Re-Os to iron meteorite evolution, the stages of evolution which can, in principle, result in Re-Os fractionation include: (a) condensation of PGE and of FeNi in the solar nebula; (b) subsequent oxidation of part of the Fe by reaction with S; (c) segregation of massive FeS during melting on parent planetesimals; (d) melting and fractional crystallization of the metal phase; (e) element redistribution during subsolidus phase transformations, including the precipitation and exsolution of schreibersite. Shen et al. have reported on analytical techniques which yield replicate analyses of Re-Os which are in good agreement, within +/-2.5 per mil. For whole samples of iron meteorites the results show a well defined correlation line on a ^187Re-^187Os evolution diagram for iron meteorites from groups IA, IIA, IIIA, IVA, and IVB, taken together. This correlation line yields a slope of 0.07848 +/- 0.00018 (2 sigma) and initial ^187Os/^188Os = 0.09563 0.00011 (2 sigma). The corresponding age is 4.61 +/- 0.01 AE using lambda (^187Re = 1.64 x 10^-11 a^-1). Re-Os data on sulphides from two iron meteorites (Group IA) show that the sulphides are extremely depleted in Re and Os. These workers concluded that sulphide formation and segregation, in the presence of FeNi, does not affect the Re-Os system (except as a diluent) and that sulphide is not useful for the possible determination of internal isochrons.