Experimental phase equilibrium data for the pseudoternary system involving (carbon dioxide + omega-pentadecalactone + dichloromethane) have been measured in order to provide fundamental information to conduct the polymerization reaction in supercritical carbon dioxide medium. The experiments were performed using a variable-volume view cell over the temperature range from 313 K to 343 K, system pressure between 3.6 MPa and 19.4 MPa and different mass ratios of dichloromethane to omega-pentadecalactone (0.5:1, 1:1 and 2:1). Phase transitions of vapour-liquid (bubble and dew point), liquid-liquid and vapour-liquid-liquid types were observed. The experimental results were modelled using the Peng-Robinson (PR) equation of state with the Wong-Sandler (PR-WS) mixing rule, providing a good representation of the experimental phase equilibrium data. (C) 2018 Elsevier Ltd.