Volume mixing ratio of CO as a function of pressure and temperature as calculated by FastChem. Overplotted are the onset of ultrahot Jupiters (as defined by their dayside temperature; Parmentier et al. 2018), the CO/CH₄ equivalency curve from Visscher (2012) as a function of pressure, the Fe condensation curve from Mbarek & Kempton (2016), and 1D temperature–pressure profiles for a hot Jupiter (WASP-39b) and an ultrahot Jupiter (WASP-18b) as computed with HELIOS (Malik et al. 2017). Both the condensation curve and the equivalency curve are computed at solar metallicity. Considering the regime of ultrahot Jupiter atmospheres, CO is a relatively stable chemical species.