The present invention relates to impurities of donepezil, or a pharmaceutically acceptable salt thereof, which may be produced during synthesis and storage of donepezil, or a pharmaceutically-acceptable salt thereof, and to methods of identifying such impurities. The invention also relates to the use of these impurities in analytical techniques and to product that contain certain low levels of these impurities in particular the following impurities are disclosed: formula (I) and formula (II). ##STR00001##