  We report here the results of a mesh experiment to measure the subpixel
structure of the EPIC MOS CCDs on board the XMM X-ray observatory. The pixel
size is 40$\mu$m square while the mesh hole spacing is 48$\mu$m, a combination
quite different from our standard mesh experiment. We have verified that this
combination functions properly and have analyzed the CCD structure with
sub-pixel resolution. The EPIC MOS CCD has an open electrode structure to
improve detection efficiency at low energies. We obtained the distribution of
various grades of X-ray events inside the pixel. A horizontally split two-pixel
event is generated near the channel stop which forms a straight vertical pixel
boundary whereas a vertically split two-pixel event is generated where the
potential due to the thinned gate structure forms a wavy horizontal pixel
boundary. Therefore, the effective pixel shape is not a square but is
distorted. The distribution of X-ray events clearly shows that the two etched
regions in each pixel, separated by the bridging finger of the enlarged (open)
electrode. We measured the difference in X-ray transmission between the
conventional and open regions of the pixel using O-K and Cu-L X-ray emission
lines, and found it to be consistent with an electrode thickness comprising
$0.2\pm0.1\mu$m of Si and $0.6\pm0.2\mu$m of SiO2.
