Photochemistry is an important phenomenon that shapes the atmospheric composition of hot Jupiters. This has been illustrated plain and clear through the detection of SO2, a photochemically produced molecule in the atmosphere of WASP-39 b. In addition, several high-resolution spectroscopy detections of HCN have been reported, which could be linked to the elemental C/O ratio of the planet. However, given the known photochemical origin of HCN, this interpretation requires accurate photochemical modelling. Here, I present recent results of pseudo-2D photochemical kinetics models of HD 209458 b and WASP-76 b, providing predictions of the HCN abundance, as well as implications carried by the recent HCN detections.