The phase diagram of small one-component particles has been analyzed under conditions of thermal insulation, i.e., conservation of energy. In large isolated systems the absolute stability belongs to heterogeneous states with phase separation. However, for small particles the global stability analysis shows a considerable extension of the single-phase regions into a two-phase zone of ?he phase diagram. Moreover, for very fine particles with sizes only 5-20 times exceeding interfacial thickness, phase separation does not occur at all and the equilibrium is achieved on homogeneous transition states that can never be obtained in bulk samples because of their absolute instability. The thermodynamic and dynamical explanations are presented. This type of a small-particle phase diagram may be relevant to the theory of amorphization, magnetocaloric effect, and nanophase composite materials where small particles or thin whiskers, capable of undergoing a transition, are immersed into a poorly conducting matrix. In case of small particles of solid solution, where mass conservation replaces the conservation of energy, present results predict the appearance of new stable phases with compositions deeply inside the miscibility gap.