Measurements have been made to 5.2 kbar at 541°C of the simultaneous diffusion of Au195 and Zn65 in ordered AuZn alloys having nominal compositions of 49.0-, 50.0-, and 51.2-at.% Zn. The activation volumes ΔV obtained from these data are maximum at 50:50 composition and decrease on both sides of stoichiometry. The value of ΔV at 50:50 is approximately 8.5 ± 0.8 cm3/mole or 0.9 times the molar volume VM, which is of the correct magnitude for a vacancy mechanism. The activation volumes for the two constituents are equal to within the experimental accuracy. The results are discussed in terms of the correlated six-jump vacancy mechanism operative in these highly ordered alloys, and are related to vacancy-defect vis-à-vis antistructure-defect structures at off-stoichiometric compositions. © 1972 The American Physical Society.