The pyrolysis of 2-phenethyl phenyl ether (PPE, C6H5C2H4OC6H5) in a hypertherrnal nozzle (300-1350 degrees C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 mu s) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C6H5C2H4-OC6H5 and C6H5CH2-CH2OC6H5 bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C6H5OC2H5) Pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 degrees C), whereas the contribution of the C6H5C2H4-OC6H5 homolytic bond scission reaction increases at higher temperatures (above 1000 degrees C).