To elucidate the fundamentals of chemical mechanical polishing (CMP) of silicon nitride, low pressure chemical vapor deposited silicon nitride is analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy both before and after CMP, demonstrating the salient increase of oxygen content, amine species, and silanol groups forming on the nitride surface. In addition, chemical testing of the collected slurry identifies ammonia as one of the nitride reaction products during CMP. Nitride hydrolysis is proposed as the dominant chemical reaction, through which the nitride surface is chemically modified and softened, enabling continuous material removal with a process that achieves atomic-scale surface smoothness. Moreover the nitride polish rate can be promoted or suppressed through the modulation of the nitride hydrolysis.