As part of the Polar Sunrise Experiment in 1998 measurements of hydrocarbons were made at the Canadian Arctic station Alert. Halogen atom concentrations play a key role in determining formaldehyde mixing ratios. Formaldehyde mixing ratios observed during ozone depletion episodes agree with those calculated from time integrated halogen atom concentrations. Formaldehyde is the most important loss mechanism for active bromine and at the same time an important source for HOx radicals. Via these reactions formaldehyde will indirectly influence chlorine chemistry and thus feedback mechanisms involving halogen atom concentrations and formaldehyde are likely to play a major role in the development of tropospheric ozone depletion episodes during polar sunrise. ¿ 1999 American Geophysical Union