Positronium yields as a function of pyridine concentration were studied in nine nonpolar solvents. Strong enhancements in the Ps yields on addition of low concentrations (<0.3 M) of the positive hole scavenger pyridine were found in those liquids where the hole moves rapidly (i.e. partly delocalized), but not in those where the hole moves slowly. Pyridine acts also as a very weak Ps inhibitor in all systems investigated if its concentration exceeds a certain limit (≈ 0.3 M). The results can be well interpreted within the framework of the spur reaction model of Ps formation.