The thermodynamic equilibrium conditions of various stratospherically relevant particles are calculated using a thermodynamic model. Due to thermodynamic constraints only certain particle combinations can exist at equilibrium in the same air mass. In particular, it is shown that liquids can coexist with solid particles over a wide range of conditions, provided that the availableH2SO4 remains in the aqueous phase. Conversely, the stability of crystalline H2SO4-containing phases is strongly affected by the formation of ice and HNO3-containing solid phases. The results are combined with laboratory data on solid phase nucleation to construct a detailed scheme for PSC formation.¿ 1997 American Geophysical Union