Concentration profiles have been determined for phosphorus diffusion into silicon. These have been fitted to the solution of Fick's diffusion equations using a model where a moving boundary separates two distinct phases. Thus, it is concluded that the silicon surface region is part of a different phase from the rest of the diffused layer. For the short diffusion times studied, the phase boundary reaction is the rate limiting process and the phase boundary moves at a nearly constant rate. In the region beyond the phase boundary the transport of phosphorus is controlled by two diffusion species, characterized by two substantially different diffusion coefficients. The slow diffusant is present mainly in a transition region between the phase boundary and the fast diffusant dominated region. The fast diffusant has a maximum concentration at the phase boundary. The linear rates of movement of the phase boundary, the concentrations of the fast diffusant at the phase boundary, and the diffusion constants for the slow and the fast diffusants were obtained over the temperature range from 820 to 1100°C. As the diffusion temperature increases the diffusion constants for the slow and the fast diffusants approach each other. At 1100°C, the diffusion profile can be represented by a single diffusion constant. For long diffusion times the phase boundary became less distinct.