Chemical reactions on sulfuric acid aerosols have recently been shown to play an important role in stratospheric chemistry. In particular, these reactions push odd-nitrogen compounds into HNO₃ and thereby enhance the chlorine-catalyzed destruction of ozone. It has been suggested that our current set of heterogeneous reactions may be incomplete. Indeed we show that formaldehyde, CH₂O, is rapidly and irreversibly taken up by stirred sulfuric acid solutions (60 to 75 wt % H₂SO₄ at −40° to −65°C) with uptake coefficients as large as γ = 0.08. If similar uptake occurs under stratospheric pressures of CH₂O (that is, 1000 times lower than used in the present study), then the removal of CH₂O from the gas phase can take away a significant source of odd hydrogen in the mid- and high-latitude lower stratosphere. We show here that with the inclusion of this reaction, concentrations of OH and HO₂ are reduced by as much as 4% under background levels of aerosols and more than 15% under elevated (volcanic) conditions. Further, the accumulation of CH₂O in stratospheric aerosols over a season may alter the composition and reactivity of these sulfuric acid-water mixtures.