The thermal stability and decomposition pathway of formamidinium iodide (FAI, HC(NH₂)₂I) in contact with NiO and TiO₂ are investigated by combined experimental studies and density functional theory (DFT) calculations. Based on the decomposition temperature, we find that the stability decreases as FAI ∼ FAI + TiO₂ > FAI + NiO. Moreover, FAPbI₃ in contact with NiO and TiO₂ shows similar thermal stability behaviour to FAI. The bulk decomposition of FAI occurs via the formation of sym-triazine, and can also produce HCN, and NH₄I at ∼280 °C, which further decomposes to NH₃ and HI above 300 °C. When FAI comes into contact with NiO, the interfacial reaction triggers decomposition at a much lower temperature (∼200 °C), resulting in the formation of NiI₂ as the solid product while releasing NH₃ and H₂O into the gas phase; sym-triazine and HCN are observed near the FAI bulk decomposition temperature. In contrast, when FAI comes into contact with TiO₂, the decomposition temperature is similar to bulk FAI; however, HCN is released at a lower temperature (∼260 °C) compared to sym-triazine. The difference in the degradation behavior of FAI with NiO and TiO₂ is elucidated using DFT calculations. Our results show that the interfacial reaction between the organic component of perovskite material and NiO occurs similarly for MA and FA, which thereby can induce device instability.