Whilst the use of Al 2 O 3 substrates by tape casting for multilayer ceramics packaging has been well established, the increasing demand for higher performance has resulted in a composite approach whereby AlN is added as a composite phase. The phase transformation in a tape-cast Al 2 O 3 /AlN composite substrate thus forms the subject of this investigation. Experimental results showed that the reaction of the two phases became significant after 1550 o C, forming a new phase AlON as sintering proceeded. Higher sintering temperatures induced a greater amount of AlON in the sintered Al 2 O 3 /AlN tapes. This third phase was due to the presence of MgO, which promoted the formation of AlON in sintered tapes even in the presence of purge gases.