The study of phase transformation in the zinc sulfide system shows that the 3C:2H equilibrium at 1020 degrees C.+ or -10 degrees C. is metastable. At 1010 degrees C.+ or -10 degrees C. the 4H polfype is the stable phase, and, from-the determination of the variation of cubic-hexagonal packing relationship with temperature, it is suggested that the polytypes all have a stability range between that of the 2H and 3C polymorphs. On this basis, the 3C:2H transition is seen to be of the diffuse first-order type. The reaction trend in the development of a polytype near the 2H:3C equilibrium temperature is 3C-->3C:2H(intergrowth)-->polytypes or 2H-->2H:3C(intergrowth)-->polytypes. If the reaction temperature is far removed from the 2H:3H metastable equilibrium temperature, then this transformation from 2H to 3C or vice versa first occurs, and thereafter the polytype-develops from the intergrowth and/or polymorph first formed. It is suggested that the nucleation and growth of the various phases is a probability function dependent on starting material, temperature and reaction time.