A cheap and easily-available organosulfur compound, guanidine thiocyanate, has been used to synthesize graphitic carbon nitride (g-C₃N₄) by a thermally-induced desulfurization and polymerization of the precursor. Photocatalytic activity experiments demonstrated that the g-C₃N₄ synthesized from guanidine thiocyanate exhibited a much higher H₂ evolution rate than the reference sample prepared from dicyanamide. Further improvement in the activity was achieved by optimization of the condensation temperature. Based on X-ray photoelectron spectroscopy analysis, the samples synthesized at a high temperature of 700 °C feature a high crystallinity and a low density of surface defects, with an enhanced optical absorbance extending to the visible light region. As a result, the sample prepared at 700 °C has been found to be 7 times more reactive towards photocatalytic hydrogen evolution than the sample polymerized at the normal temperature of 550 °C. Other guanidine halides have also been applied to synthesize g-C₃N₄ materials, and the structure and activity of the resultant g-C₃N₄ were compared to the carbon nitride polymer from guanidine thiocyanate.