Positron annihilation lifetime spectroscopy was used to study the free-volume size distribution and the o-Ps (ortho-positronium) formation in two amorphous polymers. We performed positron lifetime measurements on poly(vinyl acetate) (PVAc) in the temperature range of 84-414 K and on poly(methyl methacrylate) (PMMA) in the temperature range of 84-454 K and as a function of time (<200 h) at four temperatures (T = 84, 149, 224, and 249 K), The glass transition temperature T-g and secondary transition temperature were determined from the average o-Ps lifetime tau(3) versus temperature. The width of the o-Ps lifetime distribution was evaluated using the maximum entropy method (MELT). Analysis by the MELT program on the spectra shows that (i) the standard deviation sigma(tau(3)) Of the o-Ps lifetime distribution in PVAc and PMMA increases slightly with temperature in the glassy state, and then increases significantly with temperature above the glass transition temperature T-g; (ii) the sigma(tau(3)) in PMMA has no observable variation as a function of positron irradiation time, which suggests that the measured free-volume distribution is not influenced by the positron irradiation. The o-Ps formation probability shows large variations with temperature, thermal history, and measuring time. These effects are discussed in the framework of the spur model.