Multi-element and multilayer high-k dielectric stacks are being explored for future generation logic and memory devices. The increased material complexity can have a significant impact on the thermal stability and crystallization behavior of the stacks directly or indirectly affecting their material and electrical properties. We have studied the effect of composition and layer thickness for hafnium and rare earth based (lanthanum and dysprosium) systems on crystallization and intermixing (silicate formation). Our results show that for multilayer dielectric stacks, the thermal stability and intermixing is function of the composition and thickness of the individual layers.