In-situ alloying of polypropylene (PP)/polystyrene (PS) binary polymer blends using anhydrous aluminum chloride as a catalyst was investigated by small angle light scattering. The phase structures, morphology, and compatibilization effect in the obtained alloys during the in-situ alloying process were investigated by Rayleigh scattering. The content of compatible domains between the two phases of PP and PS in the in-situ alloys, i.e. the volume fraction of the interfacial transition layer, and the ‘invariant’ of the alloys were first calculated to describe the in-situ alloying dynamics, which reveals that the resulting in-situ PP/PS alloys are partially compatible. The relationship between the volume fraction of the interfacial transition layer or ‘invariant’ and the in-situ alloying reactive conditions are discussed in detail. The phase structural parameters, including correlation distance and average chord lengths, were calculated to characterize the phase size evolutions of the in-situ alloys, confirming the validity of using the volume fraction of the interfacial transition layer or ‘invariant’ to investigate the in-situ alloying dynamics.