A microcasting process is used to produce high aspect ratio ( > 30) monocrystalline pure aluminium wires with a diameter between 14 and 115 mu m. The role of thermal activation in the plastic deformation of these microwires is measured by means of (single) tensile stress relaxation tests. The microwires deform largely in an intermittent fashion, i.e., through repeated sudden displacement bursts, also during stress relaxation, implying that the bursts can be triggered through thermal activation. By separating the intermittent and the continuous parts of the stress relaxation load vs . time signal, we measure an activation area characteristic of the continuous relaxation mechanism. Haasen plots of the continuous relaxation data suggest the presence of a back-stress on the order of 2.5 MPa, which can be attributed to the thin layer of oxide covering the metal. Smaller (14-25 mu m) diameter crystals oriented for single slip show highly scattered activation area values and a steeper rate of increase of data in the Haasen plot than all other samples, which conform, save for the back-stress, with what is observed in bulk aluminium. Present findings are consistent with data from commensurate aluminium crystals in replicated micro cellular structures. Data of this work show that, in single slip within aluminium crystals, the coupling between the activation area and the flow stress is altered when crystals are below 30 mu m in diameter. The absence of a systematic difference in data between samples produced of 4N or 5N aluminium suggests that this conclusion is characteristic of the pure metal.(c) 2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )