Structure development in PMMA/SAN28 blends was studied experimentally with small-angle light scattering (SALS) and computationally with a diffuse-interface model based on the Cahn–Hilliard theory. All three stages of phase separation were observed with SALS. The diffusion coefficient was derived from the initial stage, while the evolution of the interface thickness was derived from all stages. The coarsening kinetics is more dominated by hydrodynamics in the late stage than in the intermediate stage of the phase separation. This is confirmed by the computational study which revealed that the coarsening becomes dominated by hydrodynamics when the capillary number is reduced from 10 to 0.5. A quantitative comparison between the experimental and numerically predicted phase separation kinetics is presented.