Hydrogen peroxide is a reactive oxygen intermediate that can play a role in a variety of redox cycles. In the ocean, it is generally considered to be dominantly photo-produced with negligible concentrations in deep waters. We have utilized a highly sensitive analytical method to investigate hydrogen peroxide in deep waters of the North Pacific Ocean. We present evidence that hydrogen peroxide exists in low nanomolar concentrations in these deep waters with an apparent minimum in the depth range of the oxygen minimum. A consideration of possible mechanisms and rates suggests both a short (~12 day) residence time as well as a biological origin for this deep hydrogen peroxide. Hydrogen peroxide is probably of minor importance to metal cycling in the deep ocean except in low oxygen environments.