A Modern Analog Technique (MAT5201/31) has been applied to fossil diatom assemblages to provide down-core estimates of February sea-surface temperatures (SSTs) and of sea ice duration over the past 220 000 years at 56°40′S, 160°14′E. At the core location, sea ice progression lagged the SST drop by ∼1 ka at interglacial-glacial transitions, and sea ice retreat was almost synchronous to the SST increase at glacial-interglacial terminations. Sea ice increased continuously during glacial periods to reach its maximum extent at the end of glacial times, although SSTs were almost constant during glacials. This indicates that SSTs are the major parameter determining the advance and retreat of sea ice at transitions, but that the sea ice advance during glacial conditions may be related to positive feedbacks of the ice on albedo, air temperature and meridional wind stress. The strong correlation (r=0.75) between sea ice duration at the core location and the Vostok CO2 record argues for a control of Antarctic sea ice extent on atmospheric CO2 concentration via the modification of the ocean-to-atmosphere gas balance.