The simultaneous occurrence of chemical and magnetic interactions in transition metal alloys greatly influences their thermodynamic properties. However the experimental evidence of the Coupling between both interactions has not been demonstrated so far. The present study shows that such a coupling exists in iron-based alloys (FeCo, FeMn, FeV) and that it modifies qualitatively the chemical short range order. This is evidenced from its temperature dependence in both ferromagnetic and paramagnetic phases which displays a marked change of behaviour at the Curie temperature (Tcm). Theoretical models which account qualitatively for the whole of the observed behaviours are briefly discussed.