The pattern formation in reaction–diffusion systems was studied by invoking the provisions contained in the moving boundary model. The model claims that the phase separation mechanism is responsible for separating the colloidal phase of precipitants into band and non-band regions. The relation between the band separation and its width are invariably related to the concentration of the reacting components. It was observed that this model provides critical condition for the band formation in semi-idealized diffusion systems. An algorithm for generating the band structure was designed, and the simulated pattern shows a close resemblance with the experimentally observed ones.