The quadruple point in carbon dioxide hydrate forming system, ice + water-rich liquid + hydrate + carbon dioxide-rich gas four-phase equilibrium condition, has been determined based on supercooled water-rich liquid + hydrate + carbon dioxide-rich gas and water-rich liquid + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions measured in this study combined with ice + hydrate + carbon dioxide-rich gas three-phase equilibrium conditions reported in the literature (K. Yasuda, R. Ohmura, J. Chem. Eng. Data 53 (2008) 2182-2188). The intersection of the three types of the three-phase equilibrium conditions was used to determine the quadruple point. The three-phase equilibrium condition experiments were performed at temperatures between 269.45 K and 275.75 K, and at pressures between 0.825 MPa and 1.708 MPa. The determined p-T condition at quadruple point was T = 271.6 +/- 0.2 K and p = 1.04 +/- 0.02 MPa. It was also observed that the freezing of supercooled water during some experimental runs performed below the quadruple point temperature, which causes sudden increase in both pressure and temperature. (C) 2016 Elsevier B.V. All rights reserved.