Hall effect measurements were made on very high resistivity polysilicon layers (around 10(5) Ohm cm) by an appropriate technique : modulation of the magnetic flux density. Accurate values of the carrier density (as low as 3 X 10(11) cm(-3)) and mobility (a few tens of cm(2) V-1 s(-1)) are deduced from the correlated function between the excitation signal and the measured voltage. It is then possible to monitor the variations of free electron concentration and free electron mobility with the concentration of dopant atoms or doping control parameter, as illustrated for low pressure chemical vapor deposited polysilicon films doped in situ over a large doping range.