A computer simulation, based on the Khachaturyan diffusion equation, is presented to develop the kinetics of the morphological evolution of the A2/B2 second-order phase transition in a binary solid solution. The evolution of the occupation probability, as a function of composition, shows a good similarity to the actual micrographs experimentally obtained based on the macroscopic composition gradient method. These results are used in parallel with the results of the evolution of the long-range order parameter as a function of the ageing time, to verify a new concept of the ordering behaviour inside the ordered phase.