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This study reports on new research into the longevity of polyester-based magnetic audio tapes. Open-reel test tapes produced 10 to 20 years ago were subjected to a variety of accelerated temperature and relative humidity ranges, chosen to best predict and assess potential conditions for storage in institutions with and without controlled environmental options. Changes to the tapes’ physical, magnetic, and chemical properties were measured to assess their impact on the tapes’ usability. The results indicated that under standard room temperature conditions, the tapes tested may remain usable for as long as 100 years, more than doubling previous estimates, a finding that could have significant implications for preservation planning of audiovisual collections.