Positron annihilation lifetime spectroscopy measurements of polystyrene and its derivatives have been performed in the temperature range 20-300 K. The ortho-positronium lifetime calculated from the lifetime spectrum analysis is related to the mean free-volume hole size. However, the ortho-positronium yield in the medium is dependent on the polymer chemistry and also on the irradiation effect induced by the positron source. We proposed a model based on two simple processes that cause, respectively, enhancement and inhibition of positronium formation to fit the experimental data over a broad temperature range. Using this model, intrinsic parameters for the polymers under study, such as the shallowly trapped electron recombination energy and the free-radical recombination energy, were calculated and discussed. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2063-2073, 2009