The first high-resolution transmission electron microscopy (TEM) cross-section images of flexible electrodes fabricated by gold ion implantation at 5 keV into polydimethylsiloxane (PDMS) are presented. A TEM sample preparation method based on cryoultramicrotomy, adapted for extremely low-modulus (1 MPa) elastomers, was developed, allowing the gold nanoparticles in a PDMS matrix to be imaged. The cluster size, size distribution and implantation depth of 50 nm were determined from the images and used to calculate the Young’s modulus of the implanted layer.