Supercritical flow experiments were conducted at University of Manitoba using supercritical flow facility-vertical (SFF-V), which is a two vertical parallel channel assembly. The working fluid was CO2 at supercritical pressure and was driven by natural convective forces rather than a pump. Different system pressures (7.4 MPa–9.1 MPa), inlet temperatures (7 °C–30.1 °C) and various outlet-channel k-factors were used. A total of eleven parallel channel out-of-phase instability boundary points were obtained and the details of those cases are reported herein. These results can be used for code validation, to enrich the limited database of supercritical parallel-channel instability and to lend further insight into supercritical flow instability in heated parallel channels.