The dilational surface rheology was applied to study the properties of adsorbed layers of a fluorinated surfactant (heptadecafluoro-1-nonanol) with a surface phase transition at the water – air interface. At relatively low surface tensions (<~57 mN/m) the dynamic surface elasticity reached anomalously high values (~200 mN/m) and exhibited a local minimum at higher surface tensions in the region of a two-dimensional phase transition. A mathematical model was developed to describe dilational properties of the adsorbed layer in the region of coexistence of two surface phases. The application of this model to solutions of the fluorinated surfactant allowed estimation of the kinetic parameters of the relaxation processes in the surface layer.