Prior measurements of hydrogen peroxide (H2O2) in Greenland ice suggested a 50% increase of the H2O2 concentration during the last 200 years, where most of the increase occurred between 1960 and 1988 <Sigg and Neftel, 1991>. In this work we present data from two shallow cores drilled at Summit, Greenland in 1995 that confirm the H2O2 increase found earlier and that show a further increase of the H2O2 concentration since 1988, leading to an overall increase of 60¿12% during the last 150 years. The new shallow cores were drilled 6 years after the Eurocore, which allowed us to identify the influence of the furnification process on the mean annual H2O2 concentration recorded in the firm. We found that the H2O2 concentration in the upper snow/firn decreased until the layer was buried with at least 1 m of snow and that the mean annual H2O2 concentrations in deeper layers stayed essentially unchanged. Besides the increase in the mean annual concentration, the annual amplitude between winter minima and summer maxima has tripled since 1970. Since there has been no significant change in temperature during either the last 150 years or last 25 years, it is unlikely that the increasing H2O2 concentrations are temperature related. We cannot rule out the possibility that seasonal accumulation patterns at Summit have changed, which could make a small contribution toward the increase. A small part of the increase of both the mean annual concentration and the annual amplitude of H2O2 in recent years could be due to increasing UV-B radiation caused by the depletion of stratospheric ozone, but a combination of changes in tropospheric chemistry apparently is involved.¿ 1997 American Geophysical Union