This paper reports on a micromechanical investigation of mudstones using a novel experimental method, based on a combination of environmental scanning electron microscopy (ESEM) and digital image correlation techniques. A specifically designed rig was developed, allowing in situ tests under combined hydric and mechanical loadings in the ESEM chamber. Observations were performed on the scale of the composite microstructure of the involved rocks (i.e. numerous grains of carbonate and quartz embedded in a clay matrix). Preliminary results from a uniaxial compression test on a sample with 5?4% water content are presented. Heterogeneous strain fields that correlate well with the microstructure of mudstones are illustrated, enabling different deformation modes (particularly shear bands and tensile microcracks) and their interactions to be identified.