The formation and properties of continuous ZSM-5 film on a porous mullite honeycomb have been investigated, The porous mullite honeycomb coated with ZSM-5 film provides a novel microporous (0.5 nm in diameter)-macroporous (0.5-0.6 mu m in diameter) composite. Amorphous silica-glass in the kaolin honeycomb sintered at 1650 degrees C is used as a source for ZSM-5 formation, The honeycomb is hydrothermally treated in NaOH, tetrapropylammonium bromide, and water to prepare a novel honeycomb-zeolite composite by in situ crystallization of ZSM-5 film. This paper describes the effects of hydrothermal conditions-such as reaction temperature, time, and concentration of NaOH solution-on the formation of ZSM-5 film on the honeycomb, and on the mechanical strength and porous properties of the honeycomb composite.