A simple model of diffusion involving the internal electric field and the presence of neutral effects was developed. The interstitial-substitutional mechanism has been found to be very sensitive to the carrier concentration in the substrate which controls the shape of the diffusion profile and accelerates or retards the diffusion process. The variations of the junction depth with substrate doping has been attributed to the equilibrium shift between interacting species rather than to the internal field. From a detailed analysis of the zinc and cadmium diffusion profiles in InP, the presence of neutrals and the interstitial charge state have been identified.