The phase equilibria in the iron rich parts of the R–Fe–Al (R=Tb, Dy, Ho, Er) phase diagrams at 500°C have been studied by X-ray powder diffraction analysis. As the aluminum contents in the series R2 (Fe1-x Alx)17 are increased the hexagonal Th2Ni17 type structure (or a substitution derivative of that structure) transforms — usually via a hexagonal TbCu type derivative structure — into the rhombohedral Th2Zn17 type structure (or a substitution derivative of that structure). The hexagonal structures dominate at or below the 2 17 molar ratio R/(Fe,Al)52/17 (10.5 and 9.5 at.% R), while the rhombohedral structure dominates at higher R contents (11.5 at.%). The solubility of aluminum in the hexagonal structure increases as the atomic size of R decreases (from x50–3.42 in Tb2Fe17-xAlx to x=0–4.75 in Er2Fe17-xAlx), while that in the rhombohedral structure decreases (from x=7.70–9.41 in Tb2Fe17-xAlxto x=5.70–7.03 in Er2Fe17-xAlx). The cell parameters suggest that the nature of the atomic substitutions in the various crystal structures changes as a 2 172x x function of both structure type and atomic size of R.