This investigation uses in situ high-resolution transmission electron microscopy to quantify the atomic-level dynamic behavior of a diffuse interphase boundary between ordered and disordered phases in an Au–Cu alloy at elevated temperature. It is found that both the interphase boundary position and thickness fluctuate with time and that the behavior of the disordered side of the interphase boundary differs from that of the ordered side. Analysis of the interphase boundary shape shows that it fluctuates by a mechanism involving the collective motion of many atoms rather than by single-atom jumps. These features are explained in terms of the physical properties of the different phases at the interphase boundary.