When fluids are confined within narrow spaces, their phase equilibria, structural, and dynamical properties become quantitatively and qualitatively different from their bulk values. The conditions obtained at thermodynamic equilibrium between a thin film and bulk fluid are derived in this paper. It is shown that the film thickness and film tension are new thermodynamic parameters. Because the thin film has properties distinct from that of the bulk fluid, it is possible for fluid to exist along planar boundaries even for systems characterized by non zero dihedral angles. For a stable thin film to exist, it is necessary that the ''disjoining pressure'' be positive or repulsive. An analysis of the disjoining pressure for a basalt melt-periodotite system indicates that a thin basalt liquid film may exist at interphase boundaries.