Abstract <p>The current state of equilibrium thermodynamics is analyzed from the standpoint of using Clausius’s initial formulation of second law δ<i>S</i> ≥ δ<i>Q</i>/<i>T</i> in describing its four fields of application: the phase states of solids,&#xa0;the states of small systems (i.e., surfaces and disperse phases), and means of statistical physics and chemical kinetics in non-ideal systems. It is noted that the symbol ≥ simultaneously reflects both the tendency to reach equilibrium in a closed system and the ultimate state of equilibrium itself. The first two applications were formulated by Gibbs in the reduced form of the second law as equality δ<i>S</i> = δ<i>Q</i>/<i>T</i>. The same sign of equality is used in developing the third application. The development of the fourth application is based on the Gibbs chemical potential. Results from using the full formulation of Clausius’s second law in each of the above four applications are discussed. It is shown that differences between the times of relaxation of thermodynamic parameters, and the self-consistent descriptions of equilibrium and kinetics in molecular approaches, must be considered in order to calculate equilibrium characteristics correctly.</p>