A typical microstructure characteristic of massive transformation such as "boundary crossing" and "irregular and jagged boundaries" was observed in an Fe-Cu alloy after water quenching and after cooling at a rate of 0.1 degrees C s(-1), which is far slower than that specified previously for the massive reaction. On the basis of the microstructure evolution and differential thermal analysis (DTA), it is shown that the massive transformation from gamma to a occurs during isothermal holding at 810 degrees C but not at 830 degrees C. During isothermal holding at 810 degrees C, the composition of the supercooled gamma phase is shifted into the metastable a one-phase region with the continuous precipitation of the Cu-rich epsilon phase. These experimental results imply that the supercooling into the one-phase region is a sufficient condition for the massive reaction to operate as long as the supercooled temperature is high enough to allow the thermally activated atomic jumps across the phase boundary.