During conventional isothermal transformation of an Fe-0.11 pct C-1.95 pct Mo alloy, eutectoid decomposition o°Curs by the interphase boundary carbide precipitation and the fibrous carbide Mechanisms at 770° to 825 °C. When proeutectoid ferrite is formed and then recrystallized within the α+ γ region, and subsequently further transformed at 770° to 825 °C, however, both of these eutectoid decomposition Mechanisms are rendered inoperative. Carbide precipitation o°Curs instead entirely as isolated particles. This result supports the deduction that carbide precipitation at austenite: ferrite boundaries can o°Cur only when these boundaries are locally immobilized by a partially coherent interfacial structure. A general approach to explaining the development of planar and curved interphase boundary precipitation, fibrous structure, and pearlite is developed in terMs of two crystallographic factors.