Two new N‐phosphorylated iminophosphoranes have been successfully obtained by Staudinger reaction using diphenylphosphorylazide and two different substituted phosphines. Synthesis, structural characterization, thermal behavior, kinetic decomposition, and DFT calculations were presented in order to establish how methyl or methoxy substituents impact on crystal packing, reactivity, chemical, and thermal stability. Therefore, the presence of the methyl group leads to the formation of supramolecular chains, while the methoxy group determines the formation of supramolecular layers with rhombohedral meshes. Comparative thermogravimetric analysis and kinetic study provide additional information on thermal stability and decomposition mechanism. The kinetic parameters were established by four different kinetic methods. The modified non-parametric kinetic method revealed a decomposition mechanism based on one predominant chemical degradation process followed by a physical transformation process. All used kinetic methods show the highest values of the apparent activation energy for the substituted methoxy iminophosphorane, which indicates the highest thermal stability. The DFT calculations study offers additional information on the reactivity and stability of the studied N‐phosphorylated iminophosphoranes by calculating the global reactivity parameters.