Positronium yields [I(3)(C)] were measured as a function of temperature in iso-octane solutions of ethyl-bromide and CCl(4) respectively at different but constant concentrations (C) of the positronium inhibitors. The quantity QC = {1/I3(C) - 1} plotted as lnQC vs. 1/T resulted in an Arrhenius type straight line. According to a simplified model the slope of the curves corresponds to the activation energy difference between the two main competing reaction pathways in the spur, i.e., solvent recombination/scavenging and positronium formation (DeltaE*(C) = E*(r,C) - E*(Ps)). The slopes were found to be positive (DeltaE*(C) < 0 and E*(Ps) > E*(r,C)), i.e., in the presence of electron scavengers higher activation energy was required for positronium formation then for the solvent recombination process. The negative value of DeltaE*(C) was increasing with increasing inhibitor concentration. This observation can be accounted for by the narrowing of the spatial distribution of the spur electrons caused by the scavengers.