A combination of measurements of electrical resistivity, Hall mobility, and phosphorus concentration by secondary ion mass spectroscopy has determined the value of the Hall scattering factor in P-doped epitaxially grown Si. The Hall scattering factor is found to increase from a value close to unity at a P concentration around 1017 cm−3, to ∼1.3 in the 1018–1019 cm−3 doping range. At higher doping levels it decreases and saturates to a value of ∼0.9 for doping levels higher than 1020 cm−3.