The stability conditions for four basic potentials have been found and analysed on the basis of the general stability theory by Gibbs for a one-component homogeneous system. The consequences of these conditions, in particular, the properties of thermodynamic systems on stability boundaries, are studied. The obtained conditions of the critical state stability lead to the conclusion that a spinodal and a binodal merge at a critical point, so the binodal can be substituted with the spinodal locally there. Some phase P–V and T–S diagrams are validated on the base of the stability conditions for the free energy and the enthalpy.