An improved method and apparatus is set forth for packaging substrates in a monolith reactor, wherein the substrates are positioned in a stacked relationship along a longitudinal axis of the reactor housing. The stack of substrates is supported in a fixed position at the bottom of the stack, and the upper end of the stack is provided with a predetermined constant compression to not only compensate for different expansions between the stacked substrates and the reactor housing, but also to hold the stacked assemblies together in a tight relationship so as to prevent deleterious vibration of the substrates. The compression is preferably applied by spring-loaded means, which may be adjusted by threading a nut against the loading spring.