A critical test has been presented to establish the nature of the kinetic pathways for the decomposition of Fe–12 at.% Si alloy below the metastable tricritical point. The results, based on the measurements of saturation magnetization, establish that a congruent ordering from B2→ D03precedes the development of a B2 + D03two-phase field, consistent with the predictions in 1976 of Allen and Cahn.