Electron mobility measurements performed at room temperature on silicon samples, heavily doped with phosphorus by thermal diffusion, are reported. Results obtained on specimens predeposited for different times, temperatures and doping gas compositions show that the electron mobility μ depends only on the carrier concentration n, and continuously decreases as this parameter increases. It is shown that the electrically inactive phosphorus and associated defect complexes inherent in highly doped diffused regions are not responsible for the observed low mobility values. An empirical relationship between μ and n, valid for carrier concentrations up to 4 × 1020cm−3, is derived. At concentrations lower than 1019cm−3, when the electron density coincides with the impurity concentration, this formula gives values in agreement with published data obtained on silicon samples doped during crystal growth.