The stable-isotope signatures of oxygen and hydrogen in the water of preserved ice and snow are both widely used to infer local temperatures of past environments. A derived quantity based on these two signatures, the 'deuterium excess' , provides additional palaeoclimatic information [2-4], as this parameter depends on the meteorological and oceanic characteristics of the water's source-regions (in particular, their temperature [2,3] and relative humidity ). Published studies mainly focus on records from the past 40,000 years. Here we present a deuterium-excess history obtained from ice cores from Vostok, East Antartica, spanning the full glacial-interglacial cycle of the past 150,000 years. The deuterium-excess record shows a strong anticorrelation with the Earth's orbital obliquity ([approximate] 41,000-year periodicity), and values are markedly higher during the cold stage 5d (following the last interglacial) than during the other cold stages. We interpret the relationship with obliquity as resulting from changes in the latitudinal insolation gradient affecting ocean surface conditions and, thus, the delivery of moisture to the polar region. We argue that the high 5d values, relative to other cold stages, are driven by relatively less moisture delivered from high latitudes, and more from low latitudes. The deuterium-excess in Antarctic precipitation thus provides long-term, spatially integrated information on ocean surface conditions and ocean/atmosphere circulations in the Southern Hemisphere.