Oxygen isotopic ratios are implemented in an ice sheet-climate model in order to directly compare the modeled isotopic ratio of the seawater to the high-resolution isotopic records from deep-sea sediment cores in the middle Miocene. The isotopic depletion resulting from the modeled ice sheet expansion explains the mean oxygen isotope step found in deep-sea sedimentary records of approximately 0.5 ppm. Approximately 85% of the modeled increase in the isotopic composition of seawater is caused by an increase in ice volume; the remainder is due to a stronger depletion in oxygen isotopes in the large ice sheet. Furthermore, we also investigated the relation between sea level (or global ice volume) and the isotopic composition of seawater. Our experiments confirm the validity of the relation of approximately 1 ppm enrichment per 100 m sea level lowering. We further show that this relationship is restricted by the mean ocean depth and the assumed oxygen isotopic composition of the ice sheet. Small deviations (+ or -10%) from this general relationship occur depending on the size and the mean isotopic content of the ice sheet. Large continental ice sheets are more depleted in heavy oxygen isotopes and therefore reach a slightly higher ratio. In contrast, small ice sheets have a less depleted isotopic composition and correspondingly have a smaller effect on the isotopic composition of the ocean.