The absolute rate constant for the reaction Cl+H2CO→HCl +HCO has been determined by the flash-photolysis-resonance fluorescence method to b 7.5¿0.9 (2&sgr;) ¿10-11 cm3 molecule-1 sec-1 at 298K and to have a negligible temperature dependence. This rate which at stratospheric temperatures is more than 2000 times faster than the rate of Cl+CH4 and more than a factor of 2 faster than Cl+HO2, indicates that formaldehyde (H2CO) will compete significantly with methane (CH4) and HO2 for the conversion of active chlorine in the stratosphere to the inactive reservoir HCl. Chlorine will thus be a less efficient destroyer of stratospheric ozone than previously believed. One-dimensional eddy-diffusion photochemical model calculations indicate that the eventual ozone depletion for a steady-state CFM release at 1975 rates (750,000 tons/year) will be lowered from 20% to 18.5% by the inclusion of this reaction.