The foraminiferal iodine-to-calcium proxy (I/Ca) can be used to reconstruct regional iodine speciation and redox conditions. This dataset contains new planktonic foraminiferal I/Ca measurements from mid-Miocene samples from ODP 202-1237, ODP 154-926, IODP 321-U1337 and IODP 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 this event.This dataset has been carefully reviewed and curated in accordance with PANGAEA's exceptionally high quality standards. However, due to discontinued communication from the authors' side, it did not receive the usual formal approval by the authors and therefore lacks final scientific validation.