Phonon scattering in phosphorus-doped silicon has been studied using a heat-pulse technique in which backscattering is measured. With crystal sizes of the order of 10 mm and with phosphorus concentrations up to 1.8*10¹⁶ cm^-3, it has been shown that single-phonon scattering is predominant. A theoretical curve has been fitted to measurements of backscattered pulse amplitude as a function of input pulse power, and from this fit the authors have been able to deduce the constant A for isotopic Rayleigh scattering in pure silicon (A=0.43 s^-1 K^-4), the deformation potential E_u associated with the bottom of the conduction band (E_u=8.6 eV), and the Bohr radius r₀ of the phosphorus impurity (r₀=16 AA). These results are in agreement with those derived by other workers from thermal conductivity measurements.