The influence of the interfacial structure on the solid state reaction products in Fe-Ti multilayers has been studied using various preparation conditions and characterization techniques. Sharp and diffused interfaces were produced by using either sequential or co-evaporation in the interfacial region. The reaction product, in the case of the sharp interface, is the bcc supersaturated solid solution of Ti(Fe) while, in the case of the diffused interface, an amorphous phase is formed. Therefore, nucleating the amorphous phase at the interface by local co-evaporation alters the reaction path observed in Fe-Ti multilayers. The solid state reactions were studied using low and high angle X-ray diffraction and Mossbauer measurements. The results are discussed in light of recent thermodynamic calculations on the Fe-Ti system.