We determined the stability of CoSiO3 relative to CO2SiO4+SiO2 with reversed experiments in a piston-cylinder apparatus. Our data at 1023, 1173, 1323, and 1473 K indicate instability of CoSiO3 below 17.2, 18.0, 19.1, and 20.4 Kbars. ΔG¿298 and ΔH¿298 (from oxides) calculated from our data between 900 and 1500 K are -12.5¿0.2 kJ and -8.0¿0.2 kJ, respectively. The dP/dT slopes of phase transformations in Fe-bearing systems in planetary interiors that are inferred from Co analogues may be incorrect.