A thorough analysis of a silicon nitride (Si3N4)-bonded SiC sidelining material from a Hall-Heroult electrolysis cell is reported. Phase composition before and after chemical degradation of the material is obtained by quantitative analysis using Rietveld refinement of X-ray diffraction data and chemical analysis. The main degradation products as a result of the oxidation of Si3N4 binder phase are Si2ON2 in the upper part and Na2SiO3 in the lower part of the sidelining. The microstructure of alpha-Si3N4 (needle) and beta-Si3N4 (shell) as well as the degradation products Si2ON2 (fiber) and Na2SiO3 (flake) were revealed by electron microprobe analysis. Chemical reactions and degradation mechanisms are proposed based on the presented findings. The degradation in the lower part is more severe than that in the upper part because Na diffusion from the cathode enhances the oxidation of Si3N4. The degradation changes the physical properties of Si3N4-bonded SiC such as density and porosity.