We report a Secondary Electron (SE) contrast modulation observed in scanning electron microscope photographs of the cross-section of SU-8 photoresist films exposed holographically. The modulation occurs along the whole depth of the sample and its contrast disappears when the samples are submitted to the post exposure bake (PEB). Diffraction and atomic force microscopy measurements of the samples were performed before and after PEB to investigate this modulation. The results indicate that this SE emission contrast modulation comes from the spatial chemical modulation generated by the photolysis during the exposure of the SU-8 films. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 226-230, 2010