This lecture delves into the Bragg-Williams analysis of order-disorder transformations in materials, focusing on the enthalpy, entropy, and long-range order parameter. It covers the determination of NAA, NBB, and NAB as functions of p and N, the critical temperature, equilibrium values of long-range order, and the molar enthalpy of mixing. The discussion also includes the short range order, total energy of order, and heat capacity around the critical temperature.