The organic-to-inorganic conversion process of cage-type poly(ethyliminoalane), (HAlNEt)n (n is mainly 8), was investigated by analyzing the gases evolved during pyrolysis. The conversion process can be divided into three temperature ranges: up to 300°C, 300-600°C, and above 600°C. Below 300°C, ethylene and ethylamine were formed probably via Al-N bond cleavage to form nitrogen terminal (≡ N:), and polymerization of cages could occur via two possible reactions involving nitrogen terminals. The conversion process between 300 and 600°C involved the evolution of ethylene, ethane and methane, whose formation can be attributed to radical reactions. Ethylamine was also formed in this temperature range. Methane formation was also observed in the temperature range above 600°C, and appears to be ascribable to the rearrangement of the mineralized network.