Aging studies with poly(ethylene terephthalate) film base indicate the life-time is equal to about 1000 years if the material is stored at 20-25°C and 50per thousand relative humidity. Concnetration changes of acid and alcohol groups that occur as a result of aging have been measured by infrared analysis. Rate constatns calculated by this method agree reasonably well with those calculated from acid contents determined by titration. Crosslinked polyester polyurethanes were prepared as models of the binder of magnetic tape. Aging studies with these materials indicate that they hydrolyze more slowly than ordinary polyester polyurethanes. Samples aged at 85°C at 100, 50, and 25per thousand relative humidity eventually deteriorated greatly in a physical sense. Magnetic tapes were aged and measurements made of the sol content of the binder and its adhesion to the polyester base. The latter quantity appears to be a more valuable indicator of tape condition than sol content. Values of binder adhesion of six brands of magnetic tape initially varied between 800 and 35 N/m (or g/cm). Binder adhesion in aged tapes was less the higher temperature and humidity of aging. A tape transport had difficulty processing tape with values of binder adhesion as low as 10 N/m. There was no problem at 35 N/m. It is anticipated that the lifetime of magnetic tapes can be predicted by measurements of binder adhesion.