Electronic Journal of Differential Equations, Vol. 2003(2003), No. 101, pp. 1-6. Title: Variational characterization of interior interfaces in phase transition models on convex plane domains Authors: Clara E. Garza-Hume (UNAM, Mexico City, Mexico) Pablo Padilla (UNAM, Mexico City, Mexico) Abstract: We consider the singularly perturbed Allen-Cahn equation on a strictly convex plane domain. We show that when the perturbation parameter tends to zero there are solutions having a transition layer that tends to a straight line segment. This segment can be characterized as the shortest path intersecting the boundary orthogonally at two points. Submitted July 15, 2003. Published October 2, 2003. Math Subject Classifications: 49Q20, 35J60, 82B26. Key Words: Phase transition; singularly perturbed Allen-Cahn equation; convex plane domain; variational methods; transition layer; Gauss map; geodesic; varifold.