Using the measured Doppler spectra of the VHF backscatter radar signals from type II ionization irregularities in the equatorial electrojet (EEJ) at Thumba (dip. 56′S), the height profiles of the phase velocity V p of the plasma waves in the EEJ are determined. It is shown that the east-west electrostatic field E y in the EEJ can be deduced from the experimental height profiles of V p using an appropriate model of ion and electron collision frequencies. The theoretical basis and the practical application of the method for deducing E y are described. The usefulness of the method even when type I irregularities are present at the higher altitudes of the EEJ is demonstrated. It is shown that the collision frequencies of ions and electrons are likely to have a significant diurnal variation, which may be caused by diurnal variations of the neutral densities and temperatures in the E-region.