Laboratory studies of formaldehyde uptake on thin sulfuric acid films indicate that HCHO can be taken up for long periods of time without saturation. Initial Henry's law solubility coefficients are large (H*=106--107 M atm-1) at stratospheric temperatures. This uptake is due to rapid formation of HCHO(aq), CH2(OH)2, and possibly HOCH2OSO2(OH) in solution. Over a longer time period, steady-state uptake continues with &ggr;≈0.002 due to formaldehyde polymerization. Although uptake by stratospheric sulfate aerosols will not provide a significant sink for atmospheric formaldehyde, it may lead to high condensed-phase concentrations of HCHO and polymerization in solution. Reaction of condensed HCHO with nitric acid in sulfuric acid has also been observed, converting HNO3 to odd nitrogen (NOx) and formaldehyde to formic acid. This reaction may have considerable impact on the HNO3/NOx ratio, as its rate is likely comparable to the HNO3 photolysis rate.¿ 1997 American Geophysical Union