The aim of this study was to understand the origin of the variation in the hydraulic reactivity of ye'elimite systems. To this end, the hydration of two clinkers, one composed mainly of synthetic stoichiometric (C(4)A(3)(S) over bar) "Y" and the other of an iron-bearing solid solution (C(4)A(28)F(0.2)(S) over bar) "Fe-Y" ye'elimite, were studied. The hydration sequences and main products were similar in both systems, but the kinetics of the Fe-11 clinker were faster. The analyses of the solution composition at early times revealed an increase of the calcium and aluminium ion concentration for Fe-Y compared to Y, whereas the sulphate ion concentration was almost identical. This resulted in an initially higher oversaturation with respect to ettringite and monosulfate for the Fe-Y clinker. Experimental results demonstrate the accelerated formation of ettringite, which explained the faster early hydration kinetics. The cause for this was the presence of small amounts of mayenite (C(12)A(7)) in the Fe-Y clinker. The impact of mayenite was verified by the addition of synthetic mayenite to the stoichiometric ye'elimite clinker. Consequently, there is no evidence that presence of iron in the ye'elimite structure nor changes in polymorphism influence the hydraulic reactivity of ye'elimite.