Abstract The increase of the positronium (Ps) yield as a function of the positron (e + ) source exposure time at 20 K was monitored in polypropylene and polystyrene by positron annihilation lifetime (PAL) and Doppler broadening (DB) spectroscopy. The contributions of the different e + and Ps states as extracted from the PAL analysis were used to reproduce the behaviour of the sharpness parameter ( S ) as a function of the wing parameter ( W ) of the annihilation line as measured by DB spectroscopy. We find a remarkable non‐linearity in the S ‐ W plots, which seems to be related to the existence of a third component (C 3 ) in a four‐component exponential analysis. We discuss the origin of C 3 in the frame of a “blob” analysis of the PAL spectra. It is shown that the simulated S‐W dependence can satisfactorily fit the observed non‐linearity only on the assumption that C 3 characterizes a e + ‐molecule complex. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)