AbstractPeroxide speciation and formaldehyde measurements have been made on ice cores retrieved from Law Dome, Antarctica. Measurements were made for ice deposited during four different periods: modern, pre-industrial Holocene, early Holocene and Last Glacial Maximum (LGM). The data show modern peroxide levels >50% above pre-industrial levels (at ∼1.6 μmol L−1) and an absence of methyl hydroperoxide (down to a detection threshold of 0.003 μmol L−1). Formaldehyde levels show a 40% increase from pre-industrial to modern times (rising from ∼0.07 μmol L−1 to ∼0.10 μmol L−1), with a further increase and possible seasonality near the surface which we associate with post-depositional processes. Peroxide levels in LGM ice are low, but formaldehyde concentrations are high (at ∼0.13 μmol L−1) relative to modern levels. Similar high levels of formaldehyde are seen in early Holocene ice (∼6900 years BP).