Model compound studies comparing rates of decomposition and product distributions from ortho- and para-phenoxydiphenylmethanes [(PhO)PhCH2Ph] suggest that hydrogen atom abstraction from the model compounds, to yield a benzylic radical intermediate, competes with hydrogen atom transfer to the aryl rings from the reduced "FeS" catalyst. A free-radical rearrangement pathway involving o-phenoxydiphenylmethane, facilitated by the presence of the "FeS" catalyst, generated in situ from ferric oxyhydroxysulfate (OHS) and sulfur, leads to apparent Ar-OAr bond scission at temperatures significantly lower than expected for homolytic scission pathways. Thermolysis of the ortho isomer proceeds predominately through a pathway involving an intramolecular addition of the benzylic radical to the 1-position of the appended diphenyl ether, Ar-1-5 participation, forming a spirodienyl radical intermediate. Scission of the C-O bond, followed by hydrogen atom abstraction, yields thermally labile o-(hydroxyphenyl)phenylmethane (oHPPM). Under the reaction conditions, at 390 degrees C, tautomerism of oHPPM to the keto isomer followed by homolysis of the weak C-C bond in the keto intermediate yields diphenylmethane and phenol. To unambiguously demonstrate the importance of the free-radical rearrangement pathway, products from the thermolysis of o-(4-methylphenoxy)diphenylmethane were quantitatively determined. Decomposition of this labeled diaryl ether at 390 degrees C in 9,10-dihydrophenanthrene containing OHS/sulfur yields 4-methyldiphenylmethane and phenol as the major products. Catalytic decomposition of the corresponding para isomer, p-(4-methylphenoxy)diphenylmethane, where the intramolecular free-radical rearrangement pathway is hindered, shows that the rate of decomposition is significantly slower than observed for the corresponding ortho isomer, and 4-methyldiphenyl ether and toluene are the major products. The selectivity observed for the product distribution in the catalytic thermolysis of the para isomer is consistent with a reversible hydrogen atom transfer pathway from the "FeS" catalyst.