When As is sequentially diffused into Ga or B‐doped Si, a retardation of thep‐type base layer is generally observed. This is in contrast to the ``emitter‐push'' effect associated with sequential phosphorus diffusions. In order to simulate transistor profiles it is necessary to be able to quantitatively describe the emmiter‐base interactions during diffusion. In this study, the way in which the internal electric field, the equilibrium vacancy density, ion pairing, and the rate of [VSiAS2] complex formation affect the redistribution of the base layer during sequential processing was investigated. Numerical solutions to coupled diffusion equations indicate that the electric field and ion‐pairing effects only cause localized retardation of a B profile during the As emitter diffusion. However, the formation of [VSiAs2] complexes causes a vacancy undersaturation in the Si to a distance in the crystal well beyond most practical collector‐base junction depths. Since the local‐base diffusivity depends upon the vacancy density, this extrinsic vacancy undersaturation effect causes the expected retarded base diffusion. Experimental verification of the correctness of the theory present is given as a function of emitter‐ and base‐surface concentrations, initial base depths, and times and temperatures.