The foraminiferal iodine-to-calcium proxy (I/Ca) can be used to reconstruct regional iodine speciation and redox conditions. This dataset contains new foraminiferal I/Ca measurements from Miocene sediments samples from ODP Site 202-1237, ODP Site 154-926, IODP Site 321-U1337 and IODP Site 321-U1338. These samples overlap with the climate event known as the Miocene Climatic Optimum. The aim of this work is to examine the geography of oxygen deficient zones in the tropical Pacific and Atlantic Oceans during the mid-Miocene.