Ratio of the abundance of OH over the abundance of H₂O as a function of equilibrium temperature from high-resolution retrievals. The orange points are from the dayside emission of the planets WASP-121 b (this work), WASP-18 b (left orange point, Brogi et al. 2023), and WASP-33 b (right orange point, Finnerty et al. 2023). The green points are obtained from the transit of WASP-76b (Mansfield et al. 2024; Gandhi et al. 2024). Points with error bars are from papers where the value and uncertainty on log₁₀(OH/H₂O) are reported (Gandhi et al. 2024, this work). For points without error bars, we take the median of the posterior of OH and H₂O to calculate the ratio. The area of the points is proportional to the surface gravity of the planets. The lines are the prediction from chemical equilibrium models computed using FastChem (Stock et al. 2022, 2018) at different pressure levels. The models used are atmospheric models of WASP-121 b, where we set the metallicity and C/O to a solar value and with a uniform vertical temperature profile at the corresponding equilibrium temperature.