In this paper, some rheological properties of fatty acid solutions at the air-water interface were studied by deformation of the equilibrium state of the solutions in the Langmuir trough in various ways. Four kinds of experiments were carried out with the same experimental setup. One of these four methods, the dilational experimental method (Pi-experimental method), has been mainly applied in this work and an accompanying theoretical model was developed. In the new model, we used a formal application for investigating the mechanisms of the sorption kinetics during the deformation of the surface under constant surface pressure by an integral equation. The new model involves two main effects : (i) the complicated flux behavior and (ii) the kinetics. The model was applied for describing the surface behavior of the decanoic acid solutions, and the agreement of the experimental results with the model is satisfactory. In particular, the comparison of the model with other standard models is instructive. The other three methods are used as complementary methods in this work.