Diffusion distances within elementally modulated reactants can be controlled on an Angstrom length scale. Below a critical repeat thickness, elementally modulated reactants interdiffuse without nucleating crystalline compounds. Using this amorphous intermediate, we can prepare metastable compounds by controlling nucleation: Above this critical repeat thickness, crystalline compounds nucleate at the reacting interfaces. Using the architecture of the initial reactant to control the diffusion distances in the initial reactant, we have found that we can prepare crystalline superlattices. Crystalline superlattice compounds containing integral numbers of inter grown transition metal dichalcogenide layers and alternating layers of transition metal carbides have been prepared through controlled crystallization of superlattice reactants with designed compositional modulation. High quality c-axis oriented dichalcogenide crystalline superlattices result from extended annealing at relatively low temperatures. A large number of [00l] diffraction orders and off-axis [10l] diffraction peaks are observed indicating that these compounds are crystalline in three dimensions. Similar annealing conditions were used to prepare carbide superlattices, however the limited low temperature diffusion rates of the carbides limit the crystallite size to approximately 300Å. The rational synthesis of these intergrowth compounds from superlattice reactants permits the exploratory synthesis of a new class of compounds and the tailoring of physical properties as a function of compositional layer thicknesses and native properties of the parent compounds.