As the dimensions of materials are reduced to the nanometer scale, changes in phase stability, referred to as pseudomorphism, are being reported. Such changes in phase stability are often serendipitously discovered in multilayered thin films. In this letter, we use a classical thermodynamic treatment to model and predict phase stability in Nb/Zr multilayers. An outcome of this letter is the development of a biphase stability diagram that represents the interrelationship of phase stability to volume fraction and length scale. Using this methodology, an hcp Nb phase stability field was empirically postulated and subsequently confirmed by x-ray and electron diffraction. The successful prediction of this phase, based upon classical thermodynamics quantities, suggests that other types of phase stabilities in other multilayers could be proposed using the biphase diagram. (C) 2004 American Institute of Physics.