The thermal explosion (TE) phenomenon was investigated in the Al-Ni system (Al-rich compositions) with various reactant medium microstructures. The parameters of TE (e.g., ignition temperature, reaction times, etc.) obtained for Al particles clad by Ni were compared with those for Al+Ni powder mixtures having different particle sizes. The heating rate was varied in the range 100 to 900 K/min, and its influence on TE characteristics was also studied. The results show that ignition process is related to either phase (melting of reactant or first product formed) or microstructural (breaking of the Ni shell in case of clad particles) transformations and depends on the reactant medium microstructure. The latter influences the process of initial product formation, which in turn affects the interaction rate during the post-ignition stages. The obtained results were discussed and compared with those reported in the literature. In general, it is suggested that TE in heterogeneous gasless systems differs from explosions in conventional homogeneous media. In the latter, the operating conditions define ignition temperature, while in the former, it is related to the system phase diagram and thus is a characteristic property of the system.