Abstract: As thin films are reduced in thickness, allotropic phase transformations to structures that are not the equilibrium phase in the standard state can be stabilized. These polymorphic phase transformations have been referred to as pseudomorphism. Many of these pseudomorphic phases have been serendipitously discovered. For the first time, the use of a classical thermodynamic model has been developed in the prediction of phase stability in Zr/Nb and Ti/Nb multilayered thin film structures. The classical thermodynamic model predicts that in regions of high volume fractions of Nb, the lower volume fraction, or alternatively, the thinner Zr or Ti layer, can be stabilized as a bcc phase rather then an hcp phase. The pseudomorphic phase is stabilized by a reduction in the interfacial free energy. An outcome of the classical thermodynamic model is a new type of phase stability diagram, referred to as a biphase diagram, in predicting which combinations of length scale and volume fraction will stabilize the pseudomorphic or bulk equilibrium phases. The change in hcp to bcc phase stability in Zr and Ti has been confirmed by transmission and reflection x-ray diffraction and electron diffraction. In each case, the Zr or Ti layer adopted a lattice parameter similar to its high temperature beta-bcc lattice parameter. An O-lattice construction, a nearest-neighbor-bond model, and a van der Merwe model have been used to estimate the contributing structural and chemical contributions to the hcp-bcc interfacial free energy reduction value. The Zr/Nb values match well to experimentally determined interfacial free energies that can be calculated from the slopes of the stability boundaries on the biphase diagram. Atom Probe Tomography (APT) results indicated a significant interdiffusion of up to 15 at.% Nb into the Ti layers that helped to facilitate the hcp-bcc transition in Ti. Refinement of the free energy calculations using the APT results brought the predicted and experimental interfacial free energy values in closer agreement for Ti/Nb. The successful prediction of bcc Zr or Ti in volume fraction rich Nb multilayers was used in the prediction and confirmation by x-ray and electron diffraction of a novel bcc to hcp phase stability change for Nb for each multilayer system. The hcp Nb phase has adopted independent hcp lattice parameters from either Zr or Ti. In either the Zr/Nb or Ti/Nb system, the hcp Nb lattice parameters were found to be equivalent. Finally, a series of Ti-8at.%V/Nb multilayers were sputter deposited. The addition of V, a bcc-stabilizer in Ti alloys, has been shown to allow for controlled facilitation of the hcp-bcc phase stability in the Ti layer. This result opens up the possibility for predictive phase engineering of multilayers at a specific layer thickness.