Formaldehyde (HCHO) measurements in snow and shallow firn at three Antarctic sites gave concentrations around 6 ppbw in surface snow and 1 ppbw and lower below 1--2 m depth. The variable concentration patterns in shallow snow and firn result from temperature-dependent uptake and release of HCHO in response to annual temperature cycles. Deeper concentrations are constant with depth, and apparently reflect average atmospheric concentrations. This implies that after accounting for differences in temperature and accumulation, changes in ice-core HCHO concentrations with depth should linearly reflect changes in atmospheric HCHO over time. Modeling of observed HCHO profiles in the snow implies that degassing of HCHO from surface snow likely contributes a significant fraction of the HCHO found in the boundary layer in spring and summer at all three sites. Based on modeling of air-snow exchange and atmospheric photochemistry, summer HCHO levels are estimated to be on the order of 100--200 pptv.