Atomic mixing by replacement collision sequences and other cascade effects is well known to create chemical disorder in irradiated alloys. Most studies of irradiation-induced disordering have focused on ex situ analysis of irradiated samples; however, fast in situ techniques are necessary to measure disordering at elevated temperatures without significant interference from concurrent re-ordering processes. In the present work, we use in situ electron diffraction with high speed data collection to measure the initial change in the long-range order parameter S with ion dose phi during 500 keV Ne+ irradiation of Cu3Au foils. The data reveal an unexpected and dramatic increase in the disordering rate as the critical order-disorder transition temperature T-C is approached. Molecular dynamics simulations show that this increase is not due to temperature-dependent cascade mixing. We attribute the enhanced disordering, instead, to coupling between point defect fluxes and the chemical state of order.