Ocean planets are volatile-rich planets, not present in our Solar system, which are thought to be dominated by deep, global oceans. This results in the formation of high-pressure water ice, separating the planetary crust from the liquid ocean and, thus, also from the atmosphere. Therefore, instead of a carbonate–silicate cycle like on the Earth, the atmospheric carbon dioxide concentration is governed by the capability of the ocean to dissolve carbon dioxide (CO 2 ). In our study, we focus on the CO 2 cycle between the atmosphere and the ocean which determines the atmospheric CO 2 content. The atmospheric amount of CO 2 is a fundamental quantity for assessing the potential habitability of the planet's surface because of its strong greenhouse effect, which determines the planetary surface temperature to a large degree. In contrast to the stabilizing carbonate–silicate cycle regulating the long-term CO 2 inventory of the Earth atmosphere, we find that the CO 2 cycle feedback on ocean planets is negative and has strong destabilizing effects on the planetary climate. By using a chemistry model for oceanic CO 2 dissolution and an atmospheric model for exoplanets, we show that the CO 2 feedback cycle can severely limit the extension of the habitable zone for ocean planets.