The Doppler broadening (DB) of positron annihilation radiation in polyethylene, teflon and silica is presented and compared with results from positron lifetime experiments. Conventional DB measurements and two detector coincidence measurements were performed. The DB spectra could be fitted to a sum of three Gaussians where the narrowest component is attributed to annihilation of p-Ps. The size of the cavities is estimated by comparing the spectra with the calculated momentum distribution of Ps in an infinite spherical well. The estimated cavity sizes derived this way are somewhat larger than those obtained from lifetime results. The deduced o-PS intensities are in agreement with lifetime results.