The extreme UV imaging telescope (EIT) on-board SOHO is performing a global survey of the extreme ultraviolet (EUV) solar corona. Operating since January 96, EIT has been producing tens thousands of images of the Sun in four narrow channels (171, 195, 284 and 304 angstrom). orbiting around the L1 Lagrangian point and oriented permanently towards the Sun, the EIT mission is a unique opportunity to study an instrument continuously exposed to solar EUV radiations. The backside thinned CCD detector is showing significant changes in its overall signal and in local 'burn in' regions. Periodic bakeouts allowed to restore a good efficiency. However, a specific observation program has been set up to diagnose the origin of the signal decay. In this framework, photon transfer analyses are performed on solar EUV images, providing good indications on the local charge collection efficiency status. Calibration lamp images are also used to eluate the signal recovery in the visible range. The signal degradation seems to be the result of two competing effects: periodic deposition of a contamination layer, and charge mobility change in the CCD Si layer as a function of the accumulated EUV dose. In this paper, the CCD quantum properties evolution is discussed, as well as the contamination issue. Preliminary diagnostics on the CCD aging under EUV radiations are exposed.