Steam reformer assembly for converting a reactor fuel into a process gas, the assembly having a catalyst therein for forming a catalyst bed having a gas flow inlet and outlet, such that the reactor fuel and steam flow in contact with the bed, the assembly including a catalyst container for the catalyst bed which has a thermal coefficient of expansion that is greater than that of the catalyst, the catalyst bed and container being subjected to temperatures and temperature cycles which are sufficient to alternately increase the volume of the container to a degree that will allow the catalyst bed to slump, and decrease the volume of the catalyst container to a degree that will subject the catalyst to crushing, comprises (a) several catalyst support assemblies for dividing the catalyst bed into several catalyst tiers, the support assemblies each consisting of a perforated base for supporting a respective one of the bed tiers, the base allowing the flow of reactor fuel and steam through the catalyst tiers and several legs which extend from the base and which support a next adjacent one of the support assemblies in the catalyst bed; and (b) the legs on support assemblies which are disposed in hotter outlet areas of the bed being shorter than the legs on support assemblies which are disposed in cooler inlet areas of the bed so as to reduce the amount of slumping and subsequent crushing of the bed in the hotter outlet areas of the bed. Also claimed is a method for minimising catalyst bed slumping and subsequent crushing which may occur during thermal cycling of a fuel gas steam reformer assembly.    USE -   Improved catalytic reactor which resists catalyst slumping and crushing.    ADVANTAGE -   The improved fuel gas reformer assembly resists catalyst slumping, and subsequent crushing, resulting from thermal cycling of the reformer is minimised, without unduly constraining the utility of the reformer, and which is suitable for retrofitting into existing reformer assemblies..