Activation volumes characteristic of room temperature compressive plastic deformation in Au- 45.2 at.% Cu- 1.7 at.% Pt, a near-AuCu alloy that orders to a fully L1(0) structure, are measured using the repeated stress-relaxation method. Four different ordering treatments (540 and 10(5)s at either 250 or 400 degrees C) are conducted on samples prior to deformation, leading to a classical two-variant polytwin structure after ordering at 400 degrees C, or alternatively a three-variant highly twinned structure at 250 degrees C. In both structures, {110} twin boundaries only a few tens of nanometres apart separate variants and constitute the main barriers to dislocation glide. All four ordered nanotwinned microstructures have an initial effective activation volume V-eff of 100-120 b(3); however, the two structures differ strongly as they work harden. The two-variant polytwin structure yields a straight line on a Haasen plot, showing that work hardening is associated with an increase in slip obstacle density, most likely dislocation debris at intervariant boundaries. The three-variant structure on the other hand shows a nearly constant (slightly decreasing) activation volume. This suggests that there is little obstacle accumulation when three variants are present, likely because plastic deformation occurs by slip of superdislocation only. (C) 2011 Elsevier Ltd. All rights reserved.