Positronium (Ps) formation on trapped electrons as a function of positron irradiation time by high-energy positrons and temperature were investigated in the case of n-heptadecane, as a representative of odd-numbered alkanes. It was found that the rise of Ps intensity with temperature in the range below 200 K results from an increasing positron mobility, at constant trapped electron density. Thermal collapse of the traps is responsible for reduction of Ps intensity at higher temperatures. (C) 2007 Elsevier B.V. All rights reserved.