A study of the interactions of ortho-positronium with a variety of inorganic and organic compounds was undertaken to investigate the mechanisms of the thermal reactions of ortho-positronium and to investigate the processes which inhibit positronium formation. It was found that the rate constants for the reaction of thermal ortho-positronium in solutions of simple inorganic salts could be correlated with the free energy change for single electron transfer, ΔG⁰, however, no such correlation was found for solutions containing a series of complexes of Hg(II), HgCl₂, and Fe(III). The rates of reaction of these complexes were found to be influenced by the nature of the ligands and the nature of the metal-ligand bounding rather than by ΔG⁰. This suggested the formation of a relatively longlived positronium complex from which the positron may annihilate. Studies of the thermal reactions of orthopositronium with substituted benzenes in benzene solution revealed that at least two mechanisms were involved, pickoff annihilation and formation of relatively longlived positronium complexes. Furthermore, evidence has been presented which indicates that inhibition of positronium formation in these systems is caused mainly by reactions of epithermal ortho-positronium. Both dissociative (Br, Cl) and non-dissociative (F) positronium capture were found to however, positron capture could not be ruled out. Certain inorganic ions were also found to oxidize epithermal positronium even if thermal reactions of ortho-positronium are energetically forbidden. The reaction of these inorganic compounds was hindered when they were complexed with large ligands.