Mechanical testing in situ scanning electron microscopy (SEM) has become a standard technique for multiscale micromechanical investigation of polycrystalline materials. Direct observation of developing strain heterogeneities allows identification of the active mechanisms and quantification of their respective contributions to the overall strain. We developed a novel experimental setup for thermomechanical testing in situ SEM, especially suited to full strain field measurements. These are based on digital image correlation (DIC), from the sample scale to the scales of the aggregate and the single grain. We present results obtained during simple compression, at controlled displacement rates and at temperatures up to 400°C, of nearly pure polycrystalline aluminum exhibiting randomly oriented coarse grains (ca. 300 m in size). Electron microlithography was applied to produce specific surface marking patterns appropriate for the different scales of interest. Full surface strain fields were obtained by digital image correlation (DIC) analysis. The localization patterns evidenced dominant crystal slip plasticity, but also substantial simultaneous and continuous activity of grain boundary sliding (GBS), the contribution of which increased with temperature. We therefore advocate that experiments such as these here presented are necessary to go beyond a description in terms of deformation mechanism maps, which attribute deformation to a single mechanism.