Although changes in the layer structure of an OLED as well as the diffusion phenomena in cathode materials and in EIM (electron-injection material) have been discussed actively, fundamental questions have remained unanswered. Using backside SIMS technique after optimised preprocessing of OLED samples, it has been possible to measure more accurate profiles of diffused elements, which was quite difficult under conventional SIMS analysis from the front surface due to so called knock-on effect. We have utilized the backside SIMS in the deterioration evaluation of OLED devices under the high temperature storage condition. Diffusion of EIM has been found with the change in layer structure depending on Tg (glass transition temperature) of HTM (hole transport material). On the basis of the analysis, it was found that the diffusion of EIM and the change in the layer structure depended on the Tg (glass transition temperature) of the HTM (hole-transport material) at the deterioration evaluation during high-temperature storage.