The order‐disorder transformation in the alloy Cu3Au has been studied at pressures ranging up to 21 kbar by means of electrical‐resistance measurements made while the sample is at high temperature and under pressure. The rate of change of the critical temperature with pressure is 2.1°K/kbar from 7 to 21 kbar. The kinetics of the order transformation belowTcare adequately described by the homogeneous reaction rate equation and an activation volume of 6.8 cm3/mole of atoms. The magnitude of this activation volume indicates that the formation of vacancies on the gold sublattice is the rate‐limiting step in the homogeneous ordering process.