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 &ggr;=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. ¿American Geophysical Union 1993