Abstract: It has been shown that on the basic of physical characteristics solid / liquid / vapor system cannot be seen as the thermodynamic. It is a kind of mechanical system. With the use of molecular-kinetic theory we obtain a new equation of the equilibrium contact angle of the following form: Cosθ₀ = (σ_A – σ_LS)/ σ_LV, where σ_A is adhesive force on the surface of a solid boundary with steam; σ_LS, σ_LV is a surface tension force of liquid at the boundaries section liquid/soiid and liquid/vapor phases respectively Based on experimental data we obtained a clear unambiguous relationship between Cosθ₀ and adhesion strength σ_A: error of test point scattering with respect to approximating line does not exceed 1%.