The first detailed structural characterisation of a functionalised tri-s-triazine derivative, trichloro-tri-s-triazine, is reported, which is a promising starting material for numerous compounds including graphitic C₃N₄ phases. DFT calculations show that a C₃N₄ structure based on tri-s-triazine should exist and that it is ∼30 kJ mol⁻¹ more stable than the previously reported C₃N₄ phase of lowest energy.