Graphite-like carbon nitride is a two-dimensional fluorescent material with discovered degradation of fluorescent properties. It was obtained by thermal decomposition of melamine and was studied using IR spectroscopy, transmission electron microscopy, and X-ray phase analysis. Three major fluorescence degradation mechanisms were identified. The first and the third ones are based on hydrogen addition to centres with an unshared electron pair, which leads to alteration of fluorescent properties and the second one is adsorption of water, which accumulates not only on the surface of the material but also probably permeates between the layers, thus leading to insignificant decrease of fluorescence.