We present results of an investigation of the details of the disordering process die to P4 annealing for Ga0.52In0.48P grown in ordered configurations with large and small macro-domain sizes. The effects of P4 annealing (710 degree(s)C) were monitored as a function of isothermal annealing time by transmission electron microscopy (TEM), photoreflectance (PR), and photoluminescence (PL). During the annealing process the band gap and PL emission energies characteristic of the disordered phase of the sample grow while those of the ordered phase decay, rather than undergoing a continuous energy change. The disordering, indicated by TEM photographs, is found to proceed downward from the free surface of the epilayer and to propagate into the bulk with a well-defined boundary rather than occurring simultaneously throughout the bulk of the layer. This disordering process is consistent with the combined results of PR and PL. The rate at which the disordering occurs is found to be much greater for samples initially having larger domains.