An attempt is undertaken to define a factor, apart from temperature, which allows for the mobility of quenched-in defects in a Cu-Al system of martensitic alloys. Experiments were performed with single-and polycrystalline alloys with nominal Al content ranging from 10 to 13.5 wt.%. An acoustic technique, operating for temperatures ranging from ambient down to 10 K, was applied to detect the existence of mobile quenched-in defects. No mobility of quenched-in defects was detected for either single-or polycrystalline alloys with Al contents close to and below the Cu3Al stoichiometric one (Cu-12.4 wt.% Al), corresponding to perfect DO3 ordering. For the alloy Cu-13 wt.% Al, mobility of quenched-in defects was detected starting from temperature of around 70 K. It is suggested that the off-stoichiometry of the alloys is accommodated through anti-site defect formation on the M-deficient side and through vacancy formation on the Al-rich side of the phase diagram. The composition dependence of damping of Cu-Al martensites exhibits a sharp maximum around the stoichiometric composition Cu3Al (Cu-12.4 wt.% Al), which is presumably formed by variations of ordered domain size and point defect concentration on both sides of the stoichiometry. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.