Lithium phosphorus oxynitride and lithium phosphate layers are prepared by controlling nitrogen composition using an optimized chemical vapor deposition process. Besides the predictable lithium phosphate layer decomposition process, progressive changes are observed in the lithium phosphorus oxynitride layer in air over time. These changes influence both the performance and stability of lithium phosphorus oxynitride, utilized as solid-electrolytes or interface barriers in batteries. Therefore, to clarify the transition mechanism of them in air, a unique experiment is designed based on x-ray photoelectron spectroscopy. The results indicate that changes in the chemical structures of lithium phosphorus oxynitride and lithium phosphate occurred alongside morphological variations. Lithium phosphorus oxynitride layers undergo steady attacks by reactive gases in air, such as O-2, CO2, and H2O, resulting in an increased number of imperfect or dangling bonds and internal chemical reactions that in turn cause morphological changes. In addition, a graphene layer is employed to reduce the reactions of lithium phosphorus oxynitride layers with reactive gases. The results show that the graphene-coated domains have relatively lower degradation rate than other regions. Overall, our results reveal the stability problems of lithium phosphorus oxynitride and lithium phosphate by demonstrating significant changes in the chemical/morphological structures exposed to air.