Oxidants are formed at the surface of Europa and may be delivered to the subsurface ocean, possibly in great quantities. Whether these substances would be available for biological metabolism is uncertain, because they may react with sulfides and other compounds to generate sulfuric and other acids. If this process has been active on Europa for much of its age, then not only would it rob the ocean of life-supporting oxidants but the subsurface ocean could have a pH of ∼2.6, which is so acidic as to present an environmental challenge for life, unless organisms consume or sequester the oxidants fast enough to ameliorate the acidification.