Paired stable oxygen isotope and Mg/Ca analyses in calcite tests of the mixed-layer-dwelling planktic foraminifer Globigerinoides ruber has been used to reconstruct equatorial Indian Ocean delta sup(18)O of seawater (delta sup(18)O sub(sw)) over the last approx. 137 thousand years. On the basis of ice-volume-corrected delta sup(18)O sub(sw) (delta sup(18)O sub(sw-ivc)), relative changes in sea surface salinity (SSS) have been estimated. The SSS estimates suggest three episodes of higher SSS (131-113 thousand years before present (kyr BP), 62-58 kyr BP, and 30-24 kyr BP) within the last glacial period as compared with the present. SSS comparison between interglacial episodes reveals that the surface seawater over the core site was significantly saltier during the penultimate interglacial than the Holocene. We suggest that the evolution of a seasonal insolation gradient between the Indian monsoon areas and the equator over the investigated time interval was instrumental in shaping the strength of the Indian winter and summer monsoons that left their imprints on the equatorial Indian Ocean SSS via freshwater input and wind-induced mixing. The study shows that the insolation difference between northern latitudes and the equator during winter affects monsoon strength in the Indian region, especially during cold intervals.