Mullite-based multilayered structures have been suggested as promising environmental barrier coatings for Si 3N 4 and SiC ceramics. Mullite has been used as bottom layer because its thermal expansion coefficient closely matches those of the Si-based substrates, whereas Y–ZrO 2 has been tried as top layer due to its stability in combustion environments. In addition, mullite/ZrO 2 compositions may work as middle layers to reduce the thermal expansion coefficient mismatch between the ZrO 2 and mullite layers. Present work studies the thermal behaviour of a flame sprayed mullite/ZrO 2 (75/25, v/v) composite coating. The changes in crystallinity, microstructure and thermal conductivity of free-standing coatings heat treated at two different temperatures (1000 and 1300 °C) are comparatively discussed. The as-sprayed and 1000 °C treated coatings showed an almost constant thermal conductivity ( K) of 1.5 W m −1 K −1. The K of the 1300 °C treated specimen increased up to twice due to the extensive mullite crystallization without any cracking.